Remove memory leak caused by label cache listeners

src/ol/renderer/Layer.js

@@ -115,6 +115,12 @@ class LayerRenderer extends Observable {
     return this.layer_;
   }
 
+  /**
+   * @abstract
+   * Perform action necessary to get the layer rendered after new fonts have loaded
+   */
+  handleFontsChanged() {}
+
   /**
    * Handle changes in image state.
    * @param {import("../events/Event.js").default} event Image change event.