Cache higher tile levels

[chrisduerr]

Currently while rendering, only the visible tiles are downloaded and drawn. Then if on a frame we didn't download any new tile, we also stage the surrounding tiles for download.

This behavior could be modified once more to also download larger tiles if neither of the previous two required any downloads. Due to the nature of tiling, larger tiles generally don't require many downloads to cache the visible area, making it well suitable for times at which the downloader is idle.