Fix computation of effective opaque region of SurfaceItemX11
The value of _NET_WM_OPAQUE_REGION can be out of sync with the geometry of the window. The NETWM spec suggests to compute the effective opaque region by taking intersection of _NET_WM_OPAQUE_REGION and the bounding region set by the shape extension.
SurfaceItemX11::opaque() got broken after refactoring opaque region handling in wayland surfaces so SurfaceInterface::opaque() always returns a sane value, the computation of effective opaque region is done in SurfaceInterface at commit time.