Item2D should render items without FBO when possible

Currently QML items inside of a 3D scene are wrapped in a Item2D node internally.  These items will render to an FBO and then just be rendered as a textured quad by the 3D renderer.

Instead it should be possible to render the 2D scenes tree directly into the 3D scene by using the Qt Quick renderer directly but changing the projection to match what the 3D scene expects.

This should then default to not using an FBO unless explictly required.

The details of this is that Qt Quick's renderer has been expanded to allow for the additional of additional content to an existing render pass.  This allows for the 3D renderer to append render commands inline while rendering it's scene such that a new instance of the 2D renderer is created and executed (similarly to how Layers are rendered anyway, but without the FBO).