added TextureCube support to DownloadStencil/DownloadDepth
added TextureCube support to DownloadStencil/DownloadDepth


Former-commit-id: 41cb71378fd0aecb37e8ff9c4fdbe9429da9dace
vrvis/aardvark.rendering,vrvis/aardvark.rendering,vrvis/aardvark.rendering,vrvis/aardvark.rendering