Alternative map generator (WIP)

[jonls]

This is an attempt at creating a map generator where the challenges are different from the original map generator. The base tile type is desert which only changes to grass around mountains and lakes. Lakes are smaller than the default maps. Deserts are relatively smooth. The map object placement is changed slightly to allow more space for buildings in grassy areas. Large deserts have small patches of grass which allows for military buildings to bridge the gap between grassy areas.

I'm not proposing to merge this, I'm just posting it here so we can discuss whether we should include alternative map generators like this at some point.