TouchpadBackend: Make sure we don't steal gestures from other instances

If we managed to register our captured event handler before another
instance it can happen that we steal the gesture from the other
instance.
E.g. the user clearly swipes horizontal but once they swiped
long enough they drifted over the vertical threshold and we
will start stealing the gesture.

Author: leolost2605

lib/Gestures/TouchpadBackend.vala

@@ -74,6 +74,14 @@ private class Gala.TouchpadBackend : Object, GestureBackend {
         event.get_gesture_motion_delta_unaccelerated (out delta_x, out delta_y);
 
         if (state != ONGOING) {
+            foreach (var instance in instances) {
+                if (instance != this && (instance.group == NONE || instance.group != group) && instance.state == ONGOING) {
+                    /* Another instance is already recognizing this gesture, make sure we don't steal it */
+                    state = IGNORED;
+                    return Clutter.EVENT_PROPAGATE;
+                }
+            }
+
             distance_x += delta_x;
             distance_y += delta_y;