Throw away the least recently used tile when the tile cache is full #45

Background

The reason finished grids are held per tile rather than per region is that an endless world has to be explorable in a fixed amount of memory. That only works if old tiles get dropped.

Description

Give the tile cache a limit on how many finished tiles it holds. When a new tile is added and the limit has been reached, drop the one that was used least recently.

Reading a tile counts as using it, not just writing one. A tile that gets read constantly must never be the one dropped.

Testing:

• Assert the tile cache never holds more tiles than its limit
• Assert adding a tile past the limit drops the least recently used one and not simply the oldest one added
• Assert reading a tile moves it to the back of the queue so it is not the next one dropped
• Assert a tile that was dropped can be generated again and put back