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/render/Event.js

@@ -17,7 +17,7 @@ class RenderEvent extends Event {
 
     /**
      * Transform from CSS pixels (relative to the top-left corner of the map viewport)
-     * to rendered pixels on this event's `context`.
+     * to rendered pixels on this event's `context`. Only available when a Canvas renderer is used, null otherwise.
      * @type {import("../transform.js").Transform|undefined}
      * @api
      */

src/ol/renderer/webgl/Layer.js

@@ -2,6 +2,8 @@
  * @module ol/renderer/webgl/Layer
  */
 import LayerRenderer from '../Layer.js';
+import RenderEvent from '../../render/Event.js';
+import RenderEventType from '../../render/EventType.js';
 import WebGLHelper from '../../webgl/Helper.js';
 
 /**
@@ -81,6 +83,36 @@ class WebGLLayerRenderer extends LayerRenderer {
   getShaderCompileErrors() {
     return this.helper.getShaderCompileErrors();
   }
+
+  /**
+   * @param {import("../../render/EventType.js").default} type Event type.
+   * @param {import("../../PluggableMap.js").FrameState} frameState Frame state.
+   * @private
+   */
+  dispatchRenderEvent_(type, frameState) {
+    const layer = this.getLayer();
+    if (layer.hasListener(type)) {
+      // RenderEvent does not get a context or an inversePixelTransform, because WebGL allows much less direct editing than Canvas2d does.
+      const event = new RenderEvent(type, null, frameState, null);
+      layer.dispatchEvent(event);
+    }
+  }
+
+  /**
+   * @param {import("../../PluggableMap.js").FrameState} frameState Frame state.
+   * @protected
+   */
+  preRender(frameState) {
+    this.dispatchRenderEvent_(RenderEventType.PRERENDER, frameState);
+  }
+
+  /**
+   * @param {import("../../PluggableMap.js").FrameState} frameState Frame state.
+   * @protected
+   */
+  postRender(frameState) {
+    this.dispatchRenderEvent_(RenderEventType.POSTRENDER, frameState);
+  }
 }
 
 const tmpArray_ = [];

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;
   }