Limit in-memory cache, so it doesn't grow indefinitely.

[podusowski]

          Additionally, I think 'cache' in `Tiles` has not correct name, or not correctly placed:

atually, it is a display cache, e.g. to display already prepared textures, not to get tile in fast way.
it would be good to limit this cache to the size of the viewport to avoid uncontrolled memory consumption

Originally posted by @noktoborus in #97 (comment)