WebGPURenderer: Fix transmission during WebXR rendering

[cabanier]

Preserve framebuffer-copy boundaries for layered array-camera rendering and replay each stage per eye so WebGPU XR samples the current view's opaque content.

Map eye-local transmission coordinates into shared camera viewports for the WebGL fallback. Add a WebGPU WebXR shadows example with transmission and its reference screenshot.

This contribution is funded by Meta

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	365.88 86.71	365.88 86.71	+0 B +0 B
WebGPU	685.67 189.48	687.12 189.84	+1.45 kB +357 B
WebGPU Nodes	683.65 189.16	685.1 189.52	+1.45 kB +366 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	510.55 124.02	510.55 124.02	+0 B +0 B
WebGPU	764.04 205.54	765.81 205.95	+1.77 kB +417 B
WebGPU Nodes	713.54 192.96	714.89 193.27	+1.35 kB +313 B

[cabanier]

The code:

• Keeps existing bundleEncoders/bundleSets.
• Renames only the stage list.
• Preserves the single-camera shader path.

cc @Mugen87