keybindings: Mask out the reserved modifiers mask

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/1144
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1219

(cherry picked from commit 61356caa)