wayland: Keep track of button modifier state

Keep track of the button modifier mask state in
ClutterInputDeviceWayland and push its state to new button events going
out.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=708781

clutter/wayland/clutter-input-device-wayland.h

@@ -53,6 +53,7 @@ struct _ClutterInputDeviceWayland
   guint                   repeat_time;
   guint                   repeat_source;
   gboolean                is_initial_repeat;
+  ClutterModifierType     button_modifier_state;
 };
 
 GType _clutter_input_device_wayland_get_type (void) G_GNUC_CONST;