Implement video texture support for WebGPU

WebGL can play video textures, and we need to implement support for this for WebGPU.

https://playcanvas.github.io/#/graphics/video-texture

See the video section here: https://toji.dev/webgpu-best-practices/img-textures.html

As this needs to use a custom sampling instruction, it might be difficult to use video texture directly from the external texture, which would be the best performance. This instruction might not be available with our GLSL->WGSL compilation. Instead, we might write a custom WGSL shader, which copies the external texture, and from then on it can be used as any other texture. The extra cost here would be the additional copy.