WebXRManager: Lazily initialise XRWebGLBinding

[harryhjsh]

Related: #31443 #30319 #30282 #31432

Description

Initialise the XRWebGLBinding as needed:

• When layers are supported by the UA
• When a granted session feature needs to make use of it (in WebXRManager)

A knock-on of this is that the Immersive Web Emulator and other webxr-polyfill consumers will work again (without needing to polyfill layers), but this is probably useful outside of that reason.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.9 79.1	338.96 79.12	+61 B +19 B
WebGPU	575.21 158.73	575.35 158.75	+132 B +26 B
WebGPU Nodes	573.82 158.49	573.95 158.51	+132 B +26 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.64 113.87	470.7 113.89	+61 B +20 B
WebGPU	646.28 174.8	646.42 174.83	+132 B +31 B
WebGPU Nodes	600.46 163.98	600.59 164.01	+132 B +30 B

[harryhjsh]

N.B. I can't test camera-access or depth-sensing at the moment.

[harryhjsh]

And just for posterity, the reason that this fixes the emulator is that Chrome doesn't implement layers (so supportsLayers == false and the binding is never created/can't throw). When/if Chrome does implement layers, the emulator/polyfilling without layers will break beyond our control (without more significant changes). Until then I feel like it's worth trying to keep it working where possible.

[mikeskydev]

I've tested camera access with this PR and can confirm everything's still working! Sorry for breaking all the emulators :-)

[harryhjsh]

Thanks for the reviews + testing!

[WORMSS]

Everywhere else uses this._sessionUsesLayers === true