Provide an API to render a QML scene to an offscreen surface

In order to easily integrate QML inside a third-party OpenGL engine, it would be nice to provide an API allowing to render the whole QML scene in an offscreen surface (FBO).

Something like QSGRenderTexture that would have the same API as QSGView maybe?
Or QSGView::setRenderTarget(Window|RenderTexture)?