add readonly

Runtime/gestures/mouse_tracking.cs

             this.annotationFinder = annotationFinder;
         }
         
-        Dictionary<int, PointerEvent> _lastMouseEvent = new Dictionary<int, PointerEvent>();
+        readonly Dictionary<int, PointerEvent> _lastMouseEvent = new Dictionary<int, PointerEvent>();
 
         public bool mouseIsConnected {
             get { return this._lastMouseEvent.isNotEmpty(); }