Remove memory leak caused by label cache listeners

src/ol/layer/Layer.js

@@ -254,6 +254,13 @@ class Layer extends BaseLayer {
     return this.renderer_;
   }
 
+  /**
+   * @return {boolean} The layer has a renderer.
+   */
+  hasRenderer() {
+    return !!this.renderer_;
+  }
+
   /**
    * Create a renderer for this layer.
    * @return {import("../renderer/Layer.js").default} A layer renderer.