Fixed Issue #462

When image filters were being applied they were cached or buffered in
`filterCanvas`. It seemed like image would be drawn multiple times (ie
calling `layer.draw()` would cause the `drawFunc` to run about 2 to 4 times).
Each time it ran it would draw on top of the cached `filterCanvas`. The
non-zero alpha values accumulated until they reached 255, effectively
removing the alpha-channel.

To fix it, I create a new `filterCanvas` everytime. I don't check for or
use the previous `filterCanvas`. I haven't notice other issues by removing
it.

src/shapes/Image.js

@@ -118,15 +118,11 @@
                 filter = this.getFilter(),
                 filterCanvas, context, imageData;
 
-            if (this.filterCanvas){
-                filterCanvas = this.filterCanvas;
-            }
-            else {
+
             filterCanvas = this.filterCanvas = new Kinetic.SceneCanvas({
                 width: width,
                 height: height
             });
-            }
 
             context = filterCanvas.getContext();