Validate the new Render Control API allows non-traditional 3D applications

It needs to be possible to use Qt Quick 3D in scenarios where a 2D window surface is not the end goal, for example when rendering to head mounted display for AR/VR devices in which there is a steroscopic view into a 3D scene.

The new rendercontrol API should be sufficient for this, but it needs to be validated.  We should create an prototype example that supports rendering to a VR headset to see if things work as intended.