Introduce support for mutiple XR Layers

[cabanier]

Description

Having support for Quad and Cylinder layers helps significantly with text readability
In #25254, I added support for multiple XR Layers but for unclear reasons, the feature was never merged.

Solution

@Mugen87 , if I update #25254 for the new renderer, do you think we can add support for XR Layers?
We could consider adding an emulation layer for browser that don't support this feature.

Alternatives

People currently hack in support for layers so it is possible, just not very elegant because it doesn't integrate with three.js' logic

Additional context

No response

[Mugen87]

@Mugen87 , if I update #25254 for the new renderer, do you think we can add support for XR Layers?

Yes, I think we can easier merge such features in WebGPURenderer because of its new character. If everything works as expected, we can backport the code to WebGLRenderer at a later point.

[cabanier]

@Mugen87 , if I update #25254 for the new renderer, do you think we can add support for XR Layers?

Yes, I think we can easier merge such features in WebGPURenderer because of its new character. If everything works as expected, we can backport the code to WebGLRenderer at a later point.

Why would that be? Wouldn't we be able to render to those layers as a separate pass? Each layer is basically a texture

[Mugen87]

There is no diff in #25254 so I couldn't see the complexity of the change. I've thought more in-depth changes are required and that would be maybe easier with the new renderer and material system. If not, it's of course okay to start with both renderers at the same time. Sorry for overlooking #25254 in the first place.

[cabanier]

There is no diff in #25254 so I couldn't see the complexity of the change. I've thought more in-depth changes are required and that would be maybe easier with the new renderer and material system. If not, it's of course okay to start with both renderers at the same time. Sorry for overlooking #25254 in the first place.

I made #30730. It's roughly the same as the previous PR.