Allow setting the source rect on QSGSimpleTextureNode

This allows the QtQuick 2D Renderer to get this information without
having to extract it from the QSGGeometry.

Change-Id: Iec99c4bc910fea9c7d0e6712a418787254a70cb2
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>
Expose a getter for the QSGEnginePrivate

This allows getting its underlying QSGContext and QSGRenderContext.

Change-Id: If0773d09b6faa360e706949757db1a6ddbe244e1
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>