This is useful when clients use subsurfaces independently of GDK.
For example if a client creates a subsurface that covers a GdkWindow
entirely. If this subsurface is opaque, Wayland compositors may not
emit callbacks for the surface of the GdkWindow any more.
Adding the covering subsurface via this new API ensures the
GdkWindow will continue to update in this case.
d3316a37ed