WebXR + WebGPU support

[dmarcos]

Worth experimenting give we have webgpu initial support on 1.7.0

https://toji.dev/2025/03/03/experimenting-with-webgpu-in-webxr.html

[vincentfretin]

One more thing to keep in mind for WebXR + WebGPU: ar-hit-test.js creates a THREE.WebGLRenderTarget and uses the synchronous renderer.readRenderTargetPixels(), which doesn't exist on WebGPURenderer (only readRenderTargetPixelsAsync). It will need the same adaptation that was done for the screenshot and reflection components in #5848 (see that PR for the pattern: feature-detect readRenderTargetPixels and fall back to the async read back). Since ar-hit-test is WebXR-only it was left untouched there, but it should be part of the WebXR + WebGPU work.

Related: renderer.xr.setPoseTarget is an A-Frame specific super-three patch to the WebXRManager used by WebGLRenderer; an equivalent will be needed in the XRManager used by WebGPURenderer (a-scene currently stubs it with a no-op, see the note added in #5847).