core: Handle Alt in the MetaPadActionMapper

In the parser we set MOD1 if we encounter <Alt>, so let's send the
respective key event here.

Closes: gnome-control-center#2593
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3522>

src/core/meta-pad-action-mapper.c

@@ -475,6 +475,7 @@ emulate_modifiers (ClutterVirtualInputDevice *device,
   } mod_map[] = {
     { CLUTTER_SHIFT_MASK, CLUTTER_KEY_Shift_L },
     { CLUTTER_CONTROL_MASK, CLUTTER_KEY_Control_L },
+    { CLUTTER_MOD1_MASK, CLUTTER_KEY_Alt_L },
     { CLUTTER_META_MASK, CLUTTER_KEY_Meta_L }
   };