meta-window-actor: Don't unredirect shaped windows

If a window has its BoundingRegion shaped, we shouldn't unredirect it,
as it expects the rest of the windows from being shown under it. This
prevents applications like the Skype screen recorder or gtkRecordMyDesktop
which want to show a "border" around the recorded area from being
unredirected, giving the appearance of making the desktop freeze.

https://bugzilla.gnome.org/show_bug.cgi?id=677657