fix(imaging): Release cached decodes, keep encoded bytes instead - #25559

Port of unoplatform#25559 (e61fa84, 98c5b27, e1e1955,
2d0e812, 6f7e0bb and d89edf0), the follow-up to unoplatform#25128, on top
of the unoplatform#25124 port.

With the bitmap cache on, the decode shared by the BitmapImages that
load the same uri and decode size was owned by an LRU of
MaxBitmapImageCacheCount (100) entries, released by no source.
- SharedImageDecode counts a shared decode's uses (the opens waiting for
  it and the sources whose subscribers draw it or were given it). Once
  it has none, it waits on PendingImageReleases like an owned decode
  (1 s and MaxUndisplayedImageBytes, oldest first), then frees its
  frames and leaves the cache. Sources are held weakly, and a collected
  source's uses are ended on the UI thread by a finalizable object only
  it holds. A subscriber back with a shared decode its source replaced
  uses it again until it draws the replacement.
- EncodedImageCache keeps the encoded bytes of images downloaded over
  http or https, bounded by the new MaxBitmapImageCacheBytes (64 MB on
  desktop, 32 MB on Android, iOS, tvOS and WebAssembly), least recently
  used first, and drops bytes unused for 5 minutes. Only successful,
  unredirected responses without no-store, no-cache or max-age=0 are
  kept. Turning EnableBitmapImageCache off drops them, checked under the
  cache's lock. AppDataUriEvaluator.DownloadAsync reports whether a
  response may be reused.
- MaxBitmapImageCacheCount is obsolete and has no effect.
- feature-flags.md describes the bitmap cache settings.
- Tests: Given_Image_Release (76 cases), GatedImageServer's extra
  header, status and redirect, and on WebAssembly a stand-in for fetch.

6.8 differences:
- As in the unoplatform#25124 port: SkiaCompositionSurface instead of
  CompositionImageSurface (a shared decode counts its frames only), the
  tests' TrackingDecoder over FrameProviderFactory, and SurfaceOf in
  ImageSource.crossruntime.cs.
- SharedImageDecode is crossruntime, like ImageSource and
  PendingImageReleases, so its surface access, byte count and release
  are Skia only. ImageData.Share is crossruntime too; Android and iOS
  never shared data.
- The decode split (ReadAllAsync, DecodeAsCompositionSurface) keeps the
  ImageSourceHelpers of the unoplatform#25542 and unoplatform#25544 ports: the browser decode
  into native memory, and MayBeAnimated from the bytes in place of
  upstream's attemptLoadingWithBrowserCanvasApi. A cached buffer is
  decoded through a read-only stream.
- Uno.UWP keeps LRUCache, which TextBlock (Android), UnicodeText (Skia)
  and Shape (native WebAssembly) still use on 6.8.

Port of unoplatform#25124's bfa20af and 129365e, with the
part of unoplatform#25559's fc608f4 that keeps a released shared decode's size.
An auto-sized Image showing a source that released its pixels, because
nothing displayed them for the release delay, laid out at 0×0 until the
source decoded them again. With the bitmap cache off, a URI downloads
again meanwhile; with it on, a released shared decode decodes again, or
downloads again when its response could not be kept. This happened to
an Image coming back to the tree, and to another Image given the
source, such as a recycled list item.

A source now keeps the pixel size of what it released until it opens
again or changes, a released shared decode keeps its own, and an Image
measures with it meanwhile, as WinUI keeps the image's size. A source
that fails or is cleared still leaves the Image empty. An Image never
added to the tree doesn't open its source, as on WinUI, and a test now
covers it.

6.8 differences:
- Arrange scaled the brush by the surface's image size, which a
  released surface, or an Image with no surface yet, doesn't have: it
  now keeps its last scale until the image is there.
- The SVG path is unchanged, as 6.8 keeps an SVG source's parsed
  document.
- 6.8 has no texture-backed surfaces, so 772c328 isn't needed.
- The size test runs on Skia desktop and the off-tree test on Skia;
  both pass on WinUI on master. The native renderers open their sources
  themselves.

Port of unoplatform#25633 (9f71cb9 and 9fcac22), with the
shared-source Image test of unoplatform#25124's fa4a240, whose helpers its
tests use.

A source that outlives its brushes, such as an icon in a static field
or in the app resources, kept every ImageBrush that ever painted it
alive, and with each one the element it painted. The source held the
brush's subscription and its Invalidated handler strongly. It now
holds both weakly: the brush keeps a lease on its subscription, whose
finalizer ends the subscription once the brush is collected, so the
source can release the pixels it owns; an open in flight keeps the
brushes waiting for it alive, but not one that left the source; and
Invalidated becomes a weak RegisterInvalidated registration.

6.8 differences:
- The open's completion also ends a shared decode's open use (the
  unoplatform#25559 port), so the keep-alive release sits inside that try.
- The native Image's WriteableBitmap handler (Image.cs) uses the weak
  registration too. The native WebAssembly brush keeps its own
  per-element subscription, unchanged.
- The test surface type is SkiaCompositionSurface.

…e (port)

Port of unoplatform#25559 commits ad534d7..9ae8529 (ad534d7,
50d0254, fd96c69, c74c86d, 526e776, 34f83de, 10f8532,
10f49ee, e81d2fe, 9ae8529), on top of this build's earlier port
of that PR.

- A BitmapImage keeps the encoded bytes it downloaded for as long as its
  UriSource stays, as WinUI's ImageCache does, so decoding it again needs
  no network. They are dropped when the source is set again (also to an
  equal Uri) or unloaded, replaced only by a result with data, and a
  source whose joined shared load fails decodes its own kept bytes.
- The encoded image cache reuses bytes only while fresh (max-age, else
  Expires less Date, less Age; no-store, no-cache, Pragma and an
  unparseable Cache-Control are not reusable), drops bytes idle for 5
  minutes on a timer, and trims above Low memory pressure from a gen 2
  callback.
- An open whose completion can't be enqueued gives up a result already
  at hand instead of leaving its shared-decode use open.

Adapted to this build: no browser Canvas API flag in the decode path,
and BitmapImage's open lives in BitmapImage.skia.cs.

Takes Uno's servicing/6.8 from 6951c2c up to 5254ace (42 pull
requests). Conflicts, by area:

IME and TextBox (unoplatform#25984, backport of unoplatform#25804; unoplatform#25952, backport of unoplatform#25619;
unoplatform#25976, backport of unoplatform#25818)
- Takes upstream's composition design (unoplatform#25804): the browser's input is
  the source of truth for the text, compositions included. This drops
  the composition parts of the unoplatform#24502 port (90746b8): its
  TypeScript composition handling, the StartIndex/TextChangePending
  event arguments, the IsReadOnly forwarding to the hidden input, and
  its Given_TextBox.BrowserSoftKeyboard tests.
- Keeps the non-IME browser fixes on top of it, in the form their
  master pull requests have on unoplatform#25804: Enter raised once per press and
  tracked by key code (unoplatform#25174, with Given_TextBox.BrowserEnterKey from
  master in place of the Enter tests that lived in the dropped file),
  the iOS refocus on tap (unoplatform#25185), pastes without text and isIOS from
  BrowserInputHelper.
- Moves the unoplatform#26005 MaxLength fixes onto unoplatform#25804's native input path, as
  master has them: a refused browser edit keeps the TextBox's selection,
  TextBoxView.UpdateTextFromNative reports whether the edit went in, and
  the IME keeps its range on the applied text (master be85699, which
  d226897 had ported onto the unoplatform#24502 IME). Adds that commit's browser
  and platform-applied IME tests.
- TextBox.pointers.skia.cs: upstream's file (unoplatform#25619 iOS touch editing,
  unoplatform#25818, unoplatform#25804), with the unoplatform#25126 port (the hold acts when it starts,
  a touch right-tap keeps the caret) in the form master's unoplatform#25126 branch
  has it on unoplatform#25619, including focusing before the iOS hold timer acts
  and its iOS not-focusable test.

Pointers (unoplatform#25976, backport of unoplatform#25818)
- Tapped and RightTapped are raised once the PointerReleased bubbled
  (unoplatform#25818), with the unoplatform#25732 port's window coordinates (2-argument
  routed event args, RawPosition for ContextRequested), as master's
  unoplatform#25732 branch has them.
- The release wrapper of unoplatform#25818 keeps the unoplatform#25126 port's
  StopContextMenuTimer at the start of the release.

DependencyObject (unoplatform#26142, backport of unoplatform#26123)
- Takes upstream, then the code of unoplatform#25839 (97a2f17), which replaces
  this branch's unoplatform#25102 ports (800247f, be41da6, fd40bf9,
  7546379). DependencyObjectStore.Binder.cs, .alc.cs,
  Application.Alc.cs, ResourceDictionary.alc.cs and their tests now
  match unoplatform#25839; this drops 800247f's own
  SingleParentNonFE_Parent_Removes_Value_Then_Next_Parent_Starts_Clean
  test, which unoplatform#25839 doesn't have.

Imaging (unoplatform#25597, unoplatform#25915, unoplatform#25911, unoplatform#25897)
- Keeps this branch's side: it has the later forms of these fixes
  (unoplatform#25124, unoplatform#25559, unoplatform#25633 and unoplatform#26370 ports), where upstream has the
  6.8-only unoplatform#25597 design and earlier snapshots. Upstream's shared
  download, IgnoreImageCache and failed-load fixes are covered by
  SharedImageDecode. Adds upstream's When_Source_Outlives_Brushes test.

ToolTip (unoplatform#26098, backport of unoplatform#25137 and unoplatform#26096)
- Keeps this branch's unoplatform#25137 port and adds unoplatform#26096 in master's form: the
  service releases only a ToolTip it opened, and leaves one the app
  opened during the show delay to the app. Keeps both sides' tests.

Flyout (unoplatform#26062, backport of unoplatform#26060)
- This branch had the same commits already; keeps the "Keep
  non-resizable flyouts in window" port and its tests.

Also removes a helper pair in Given_AccessibleTextBox that both sides
added.