add WebGPU XR optimization

[cabanier]

• Add native WebGPU XR view-instanced rendering when projection-layer sub-images share a texture array. The WebGPU XR bridge now detects compatible layered sub-images, records the view-count state on the device, and drives FramePassMultiView through a single native pass when supported.
• Request and expose the WebGPU view-instancing and multisampled-array-textures capabilities, including maxViewInstanceCount handling. Backbuffer render-target setup now allocates array-layered color/depth attachments for native view-instanced XR, including multisampled texture-array attachments when needed.
• Teach WGSL processing and bind group layout generation about multisampled 2D array textures, native view-index handling, and view-uniform arrays. Materials and mesh shader caching now include the view-instancing shader variant so XR array uniforms select the correct per-view matrices and camera data.

[mvaligursky]

Amazing, I'll look into details next week, thanks so much!

[mvaligursky]

I think we should move the built-ins / clustered lighting changes to a separate PRs, as those are pretty much read to be merged. I'll do it next week, unless you do before then. Great changes overall. Any idea where we might get those other multi-view / multi-sampled arrays extensions?

[cabanier]

I can move those to their own separate PR.

[cabanier]

They will land as experimental features in the next 2 releases of Quest Browser. As soon as the proposals are merged, I will ship them by default.

[cabanier]

@mvaligursky done. I moved the optimization to a different patch. I'll keep this one as "draft"