Introduced QQuickCustomRenderStage.

The purpose of this concept is to make it possible to plug in an
external rendering stage, like replacing the GL rendering with a
composition step performed by a hardware compositor in QtCompositor.

Change-Id: I994b93af443f68a77ca73cf310b5910c49e014c3
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>