Update PenSkin.js

src/PenSkin.js

@@ -154,6 +154,13 @@ class PenSkin extends Skin {
         return true;
     }
 
+    /**
+     * @returns {boolean} true if alpha is premultiplied, false otherwise
+     */
+    get hasPremultipliedAlpha () {
+        return true;
+    }
+
     /**
      * @return {Array<number>} the "native" size, in texels, of this skin. [width, height]
      */