xwm: Fix xwm opaque region

xwm: Fix xwm opaque region

Since surface.commit was introduced, opqaue regions are stored in a pending
variable that isn't used until surface.commit. Xwayland uses the surface opaque
region as a way to tell weston what region of the surface should be opaque.
However when this pending opaque region was introduced, xwm was not updated
and so we have the 'black = transparent' problem again. This patch fixes the
problem by having xwm use the pending opaque regions.