It should be possible to use QQuickRenderControl to render without an FBO

Right now QQuickWindowPrivate::renderSceneGraph() can operate either targeting a window or an FBO. There is a third case however: when one uses QQuickRenderControl but does not want to provide an FBO and just want the Quick rendering to go into whatever context/surface is current as-is. (the size still has to be provided)

In fact passing in an FBO ID of 0 should mostly work. Needs testing.