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

[rgerd]

This issue comes from https://github.com/BabylonJS/Babylon.js/pull/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.

[thomlucc]

@RaananW - can you assign the correct milestone?

[github-actions[bot]]

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