perf(wasm): Create image decode workers on demand (backport #25132) - #25897

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.