WebXRManager: Inherit active layers from camera

[mrxz]

Fixed #24354

Description

This PR updates the WebXRManager to copy over the layers mask to the XR cameras. The respective eye layer (1 for left, 2 for right) is always enabled in addition.

• A default scene setup behaves identical to before
• Calling renderer.render(scene, camera) now always respects camera.layers whether XR is active or not
• Users that manually enabled layers on the XR cameras will now have to enable them on the main camera instead.
• Users will have to be careful with layers 1 and 2 as these are used for the eyes (even before this PR)

This contribution is funded by Fern Solutions

[github-actions]

��� Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	691.57 171.41	691.6 171.42	+26 B +9 B
WebGPU	818.87 220.85	818.87 220.85	+0 B +0 B
WebGPU Nodes	818.38 220.72	818.38 220.72	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	463.72 112.16	463.75 112.17	+26 B +8 B
WebGPU	539.75 145.78	539.75 145.78	+0 B +0 B
WebGPU Nodes	495.74 135.6	495.74 135.6	+0 B +0 B

[Mugen87]

/cc @cabanier

[jrjdavidson]

Just in case it's useful, see #26340