Fixed flick gesture.

src/trackers/transform.py

@@ -185,7 +185,7 @@ class TransformationTracker(GestureTracker):
             prev = self.update_centroid()
 
             if not self.points:
-                self.trigger(FlickGesture(event, point, point - prev))
+                self.trigger(FlickGesture(event, point, point - prev.prev))
 
             if not self.propagate_events:
                 event.stop_propagation()