window-actor/x11: Update opaque region

Merged Olivier Fourdan requested to merge ofourdan/mutter:opaque-region into master

Now that the opaque region is not reset by the Wayland actor surface, we need to update the opaque region just like we do for input region.

That fixes a regression with client-side decoration X11 windows running in Xwayland.

Closes: #1043 (closed)

See also: !1058 (merged)

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