meta_display_process_key_event() always looks up events based on the
"default" keysym for the keycode, so we should do the same here. This
fixes, eg, the lookup of Shift-Alt-Tab (which would otherwise be
unrecognized because the keysym would be XK_ISO_Left_Tab rather than
XK_Tab).
https://bugzilla.gnome.org/show_bug.cgi?id=632155