feature : zoom : continued

- simplified Renderer(s) architecture (removed decorator,
  CachedFrameRenderer simply inherits from FrameRenderer now)
- keeping AbstractRenderer to act as interface
- fixed issue with layers : forgot to clone the first frame while merging
  and therefore was modifying the original frame when I just wanted to
  create a tmp frame (FrameUtils.js)
- extracted the mousemove throttling delay used in DrawingController to
  Constants.js and reduced it from 40ms to 10ms

js/Constants.js

@@ -48,5 +48,6 @@ var Constants = {
   GRID_STROKE_WIDTH: 1,
 
   LEFT_BUTTON : 'left_button_1',
-  RIGHT_BUTTON : 'right_button_2'
+  RIGHT_BUTTON : 'right_button_2',
+  MOUSEMOVE_THROTTLING : 10
 };