There were two issues with using the shape region to derive an input
region.
Firstly, the shape region is against the client rectangle, while the
surface actor needs it to be against the buffer rectangle. Fix this by
offsetting the shape region before passing it along.
Secondly, we can't just intersect the shape and input region, since that
leaves out the window decorations. Fix this by only intersecting the
input region covering the client part, and the shape region, and then
union that with the input region covering the rest.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3045>
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