scale gesture

Author: zhuxingwei

Runtime/gestures/constants.cs

@@ -9,6 +9,8 @@ public static class Constants {
         public const float kDoubleTapSlop = 100.0f;
 
         public const float kPanSlop = kTouchSlop * 2.0f;
+        
+        public const float kScaleSlop = kTouchSlop;
 
         public static readonly TimeSpan kPressTimeout = new TimeSpan(0, 0, 0, 0, 100);