csd: Fix invisible border/shadow confusion

csd: Fix invisible border/shadow confusion

We did not set an input shape on the window, so the region outside
the invisible border where we draw the outer edges of the shadow
were still part of the window, as far as clicks and cursors were
concerned. Fix this by setting an input shape that makes all clicks
outside of the resize borders go through to the underlying window.

https://bugzilla.gnome.org/show_bug.cgi?id=726125