Basic structure may work. It may lack the begin/endExternal calls, however.
examples/quick/scenegraph/fboitem needs porting to be able to verify.
Does not cover cases when using Vulkan, Metal, or D3D. There QQuickFbo will have no effect. But in order to be able to keep the QQuickFbo API as an OpenGL-only thing, we should make it functional in combination with the OpenGL backend of QRhi.