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
      */