Add prerender and postrender events for Image layer

src/ol/renderer/canvas/ImageLayer.js

@@ -137,10 +137,12 @@ class CanvasImageLayerRenderer extends CanvasLayerRenderer {
     const dw = img.width * transform[0];
     const dh = img.height * transform[3];
 
+    this.preRender(context, frameState);
     if (dw >= 0.5 && dh >= 0.5) {
       this.context.drawImage(img, 0, 0, +img.width, +img.height,
         Math.round(dx), Math.round(dy), Math.round(dw), Math.round(dh));
     }
+    this.postRender(context, frameState);
 
     if (clipped) {
       context.restore();

src/ol/renderer/canvas/VectorImageLayer.js

@@ -105,6 +105,16 @@ class CanvasVectorImageLayerRenderer extends CanvasImageLayerRenderer {
     return !!this.image_;
   }
 
+  /**
+   * @override
+   */
+  preRender() {}
+
+  /**
+   * @override
+   */
+  postRender() {}
+
   /**
    * @inheritDoc
    */