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.

BUG: 455617


(cherry picked from commit 35a3184e)