gesture-action: unregister point on missed release events

When we miss button release events (eg. when they happen outside
of our window) we must ensure that the corresponding point is
removed from the array of tracked points, otherwise GestureAction
will get very confused and will cancel all subsequent gestures.

https://bugzilla.gnome.org/show_bug.cgi?id=683471

clutter/clutter-gesture-action.c

@@ -226,6 +226,7 @@ stage_captured_event_cb (ClutterActor       *stage,
         if (!(mods & CLUTTER_BUTTON1_MASK))
           {
             cancel_gesture (action);
+            gesture_unregister_point (action, position);
             return CLUTTER_EVENT_PROPAGATE;
           }
       }