Use post render frame for tile pruning

We should discuss whether post render functions must be run after each render frame or not.  If these can be run after multiple render frames, it would make sense to increase the timeout.  As it is, it looks like post render functions are run for every render.  Hard to see what the benefit is in this case.

src/ol/renderer/canvas/canvasvectorlayerrenderer.js

@@ -334,7 +334,7 @@ ol.renderer.canvas.VectorLayer.prototype.renderFrame =
   this.renderedExtent_ = tileRangeExtent;
   if (!this.pendingCachePrune_) {
     this.pendingCachePrune_ = true;
-    goog.global.setTimeout(goog.bind(this.pruneTileCache_, this), 0);
+    frameState.postRenderFunctions.push(goog.bind(this.pruneTileCache_, this));
   }
 
 };