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/utils/FrameUtils.js

@@ -5,7 +5,7 @@
     merge : function (frames) {
       var merged = null;
       if (frames.length) {
-        merged = frames[0];
+        merged = frames[0].clone();
         var w = merged.getWidth(), h = merged.getHeight();
         for (var i = 1 ; i < frames.length ; i++) {
           pskl.utils.FrameUtils.mergeFrames_(merged, frames[i]);