WebGPURenderer: Persistent video texture approach

[sunag]

Closes #31330, #31397

Description

Replaces texture_external with a persistent texture using copyExternalImageToTexture() updating by requestVideoFrameCallback of VideoTexture, reusing all of the core's texture management, including sharing between materials, and adds support for vertex stage usage too.

@Mugen87 I think this is the best way, in addition to the benefits, it reduces the total code.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.28 78.93	338.28 78.93	+0 B +0 B
WebGPU	558.52 154.58	557.63 154.34	-893 B -248 B
WebGPU Nodes	557.45 154.37	556.55 154.12	-893 B -249 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	469.62 113.66	469.62 113.66	+0 B +0 B
WebGPU	634.18 171.62	633.28 171.4	-892 B -220 B
WebGPU Nodes	589.31 160.95	588.42 160.71	-892 B -239 B

[Mugen87]

Yes, that sounds like a good solution 👍 .

Are parts of the PR missing though? I don't see any changes to VideoTexture or usage of copyExternalImageToTexture()?

[sunag]

Great, right? The current texture system was reused, and most things worked auto-magically.

[Mugen87]

Ah I see, the code goes now in the WebGPUTextureUtils._copyImageToTexture() branch. Neat!