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