When switching layouts, special modifiers bits may be be set for
internal use by Xkb.
As we now ignore a set of modifiers when processing the special
modifiers keys, we ought to also mask out those reserved modifiers
otherwise we would discard the [Super] key after switching layouts
in X11.
Closes: https://gitlab.gnome.org/GNOME/mutter/issues/1144https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1219
(cherry picked from commit 61356caa0693f19ccb289597bbab5138319e7f3c)