Set format parameter to rbush function

To remove the following closure compiler warning:
```
Function module$node_modules$rbush$index.default: called with 1 argument(s). Function requires at least 2 argument(s) and at most 2.
```

src/ol/renderer/canvas/VectorTileLayer.js

@@ -66,7 +66,7 @@ const CanvasVectorTileLayerRenderer = function(layer) {
    * Declutter tree.
    * @private
    */
-  this.declutterTree_ = layer.getDeclutter() ? rbush(9) : null;
+  this.declutterTree_ = layer.getDeclutter() ? rbush(9, undefined) : null;
 
   /**
    * @private