Render-to-texture widgets' closing do not update the containing window

Open the quickwidget example and hit the X to close the subwindow with the QQuickWidget. The subwindow stays visible.

When disabling SwitchableWidgetComposition (i.e. sticking with GL-based compositing for ever), the problem is not there, the window is composed and flushed correctly.

It may have to do something with dirty areas when no more rtt widgets are visible and switching back to the normal flush path.