Currently mLayerCount for RenderTarget is always updated to the number
of depth for attachments, which incurs unintended behaviors for some
types of textures. i.e., 2d array, cubemap array, and 3d textures.
Fix this by updating mLayerCount only for multiview use case.
BUGS=[369165616]
Clients can create a multi-layered render target that consists of array
textures, and use it as a custom render target.
A new sample app "hellostereo" demonstrates how to use this feature.