CompressedTextureLoader: Return cube maps as cube textures. - #35084

Description

CompressedTextureLoader returns cube maps (DDSLoader, PVRLoader) as a CompressedTexture with the faces in image, without flagging it as a cube texture. Code that checks isCubeTexture handles it as a 2D texture:

• WebGLRenderer: as scene.background it is bound as a 2D texture, and WebGLTextures throws "Cannot read properties of undefined (reading 'length')" every frame. As envMap of a material that samples it directly, like MeshBasicMaterial, it is mirrored compared to CubeTexture, to CompressedCubeTexture and to the same texture on MeshStandardMaterial, since flipEnvMap depends on isCubeTexture and PMREM doesn't.
• WebGPURenderer decides between 2D and cube textures by isCubeTexture, so it can't use these cube maps at all.

Fiddle (WebGLRenderer): https://jsfiddle.net/zqnj6Lts/6/
A DDS cube map as background and as envMap of two spheres. The canvas stays black. With params.background = false, the left sphere (MeshBasicMaterial) is mirrored compared to the right one (MeshStandardMaterial).

	dev	this PR
background: true
background: false

Whether the data hold a cube map is only known once the file has been parsed, so this PR turns the texture returned by load() into a compressed cube texture at that point: it sets isCubeTexture and isCompressedCubeTexture, as CompressedCubeTexture does, and CubeReflectionMapping unless the app has set a mapping. With the CompressedCubeTexture PR, WebGPURenderer renders these cube maps too.

The cube map reflections in webgl_loader_texture_dds are no longer mirrored, so this PR updates its screenshot. webgl_loader_texture_pvrtc changes the same way where PVRTC is supported; its screenshot stays, since the E2E browser doesn't support PVRTC.