What's more, demonstrate two types of rendering to a cubemap:
one by one to each face, and by attaching all faces as color
attachments in one go.
Both are used by Qt Quick 3D in connection with shadows, so this
proves that the same is possible to implement with QRhi.
Task-number: QTBUG-81261
Change-Id: I5c7077224d7cae0dd6ea02ac30a9e6f9f1f0c229
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
