Implement preRender and postRender methods for WebGLLayerRenderer

This is required so that Renderers like WebGLPointsLayerRenderer can fire prerender and postrender events.
Problem described in [issue 11463](https://github.com/openlayers/openlayers/issues/11463).

src/ol/renderer/webgl/PointsLayer.js

@@ -409,6 +409,8 @@ class WebGLPointsLayerRenderer extends WebGLLayerRenderer {
    * @return {HTMLElement} The rendered element.
    */
   renderFrame(frameState) {
+    this.preRender(frameState);
+
     const renderCount = this.indicesBuffer_.getSize();
     this.helper.drawElements(0, renderCount);
     this.helper.finalizeDraw(frameState);
@@ -425,6 +427,8 @@ class WebGLPointsLayerRenderer extends WebGLLayerRenderer {
       this.hitRenderTarget_.clearCachedData();
     }
 
+    this.postRender(frameState);
+
     return canvas;
   }