Trimming the tile cache can have a high overhead, since we need to
scan the tile list for an eligible/worthwhile tile to trim.  If
trimming happens frequently, we end up re-scanning the list each
time.

In order to amortize the cost of scanning the tile list, over-trim
the cache by a certain amount, instead of only trimming the excess.
When trimming happens frequently, we progressively increase the
over-trimming ratio, up to 50% of the cache size limit.