WebGPURenderer: Add Video and Storage 2D Texture textureLoad Support

[RenaudRohlinger]

Description
https://www.w3.org/TR/webgpu/#external-texture-sampling

External textures are represented in WGSL with texture_external and may be read using textureLoad and textureSampleBaseClampToEdge.

https://www.w3.org/TR/WGSL/#textureload

This PR adds support to read a single texel from a video or storage texture in the vertex and compute shading.

This contribution is funded by Utsubo

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.14 78.99	339.14 78.99	+0 B +0 B
WebGPU	484.61 134.47	484.69 134.48	+74 B +10 B
WebGPU Nodes	484.08 134.37	484.15 134.38	+74 B +11 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	464.62 111.98	464.62 111.98	+0 B +0 B
WebGPU	553.37 149.76	553.45 149.77	+74 B +9 B
WebGPU Nodes	509.25 139.47	509.33 139.48	+74 B +10 B