Prove that integrating bigger external graphics frameworks is feasible in Qt Quick on QRhi on OpenGL

Explore bigger external frameworks with custom OpenGL rendering, for example Qt Quick 3D and Qt 3D. Do those work with the RHI and the OpenGL backend? (Scene3D needs an RHI capable version at least)

Essential for the Qt 6 story, we will end up in trouble if these existing solutions cannot integrate with Qt Quick anymore, not even when running with the OpenGL backend of the RHI.