Introduced QQuickCustomRenderStage.

The purpose of this concept is to make it possible to plug in an external
rendering stage. The primary usecase right now is to swap out the GL
rendering with a hardware compositor pass in the wayland module.

Change-Id: I994b93af443f68a77ca73cf310b5910c49e014c3

Conflicts:
	src/quick/items/qquickwindow_p.h
	src/quick/scenegraph/qsgthreadedrenderloop.cpp