Add support for non-multiview rendering with texture-array composition layers

Feature requests
1

This issue comes from #11828

We currently can’t explicitly associate a RenderTargetTexture with a layer index in an underlying texture of type TEXTURE_2D_ARRAY. This is necessary to be able to render to specific eyes in composition layers initialized with textureType = 'texture-array'. Perhaps we could subclass RenderTargetTexture with a type that calls engine.bindFramebuffer with the layer associated with it–in the case of composition layers this would be the subImageIndex in the XRWebGLSubImage returned from XRWebGLBinding.getSubImage.

*Ported from GitHub