Very large maps

[bcvery1]

Indications are that a very large map (1000x1000 tiles) causes unexpected behaviour.

Need first to establish if this is reproducable, and if so, roughly how many tiles are needed to cause it?

Need then to get a fix - either logging a sensible warning in the event of a user loading a map which is likely to cause problems; or, ideally, finding the bottleneck and fixing it.

ultimately, there will be a limit on map size, which is going to be dependent on many things outside of this library