Tile loading is slow to start

Since #437, a maximum of two new tiles are loaded per frame. This delays loading when a large number of new tiles must be loaded:

* When the map is initially rendered
* If the user radically changes the view (e.g. by changing center or resolution)

For more information, see [@elemoine's comment](https://github.com/openlayers/ol3/pull/437#issuecomment-15957286).