Update Skin.js

src/Skin.js

@@ -76,6 +76,13 @@ class Skin extends EventEmitter {
         return false;
     }
 
+    /**
+     * @returns {boolean} true if alpha is premultiplied, false otherwise
+     */
+    get hasPremultipliedAlpha () {
+        return false;
+    }
+
     /**
      * @return {int} the unique ID for this Skin.
      */