There are still tests failing, but this change addresses some of the new failures after 4b163e0482 (which is an awesome improvement despite the test failures :).
Having the TileManager remove an image from the DOM, then setting the
cached image, and then having to position it felt a bit awkward. With the
new beforeload event, the setImage method and putting renderTile before
positionTile, providing the cached image feels way more natural.