More cleanup in the WebGL tile layer's dispose method

src/ol/layer/Layer.js

@@ -348,6 +348,11 @@ class Layer extends BaseLayer {
    * Clean up.
    */
   disposeInternal() {
+    if (this.renderer_) {
+      this.renderer_.dispose();
+      delete this.renderer_;
+    }
+
     this.setSource(null);
     super.disposeInternal();
   }

src/ol/layer/WebGLTile.js

@@ -255,6 +255,9 @@ function parseStyle(style, bandCount) {
  * property on the layer object; for example, setting `title: 'My Title'` in the
  * options means that `title` is observable, and has get/set accessors.
  *
+ * **Important**: after removing a `WebGLTile` layer from your map, call `layer.dispose()`
+ * to clean up underlying resources.
+ *
  * @extends BaseTileLayer<import("../source/DataTile.js").default|import("../source/TileImage.js").default>
  * @api
  */
@@ -320,4 +323,11 @@ class WebGLTileLayer extends BaseTileLayer {
   }
 }
 
+/**
+ * Clean up underlying WebGL resources.
+ * @function
+ * @api
+ */
+WebGLTileLayer.prototype.dispose;
+
 export default WebGLTileLayer;