Update RenderWebGL.js

src/RenderWebGL.js

@@ -2317,6 +2317,13 @@ class RenderWebGL extends EventEmitter {
         this._snapshotCallbacks.push(callback);
     }
 
+    /**
+     * Get the DrawableId of the pen layer
+     */
+    getPenDrawableId () {
+        return this._allDrawables.findIndex(drawable => drawable._skin._id === this._penSkinId);
+    }
+
     /**
      * Update the list of custom fonts. These fonts will be added to the DOM.
      * SEURITY CONSIDERATIONS: It is the caller's responsibility to ensure that the @font-face