Added createImageData(ImageData) support

lib/context2d.js

@@ -469,15 +469,20 @@ Context2d.prototype.getImageData = function(x, y, width, height){
 };
 
 /**
- * Create `ImageData` with the given dimensions.
+ * Create `ImageData` with the given dimensions or
+ * `ImageData` instance for dimensions.
  *
- * @param {Number} width
+ * @param {Number|ImageData} width
  * @param {Number} height
  * @return {ImageData}
  * @api public
  */
 
 Context2d.prototype.createImageData = function(width, height){
+  if (width instanceof ImageData) {
+    height = width.height;
+    width = width.width;
+  }
   var arr = new PixelArray(width, height);
   return new ImageData(arr);
 };