improve forest growth

[anyweez]

Forests should be biome-aware. There are two types of forest biomes, which should have denser tree growth.

Look into Poisson disk sampling for this use case:
http://devmag.org.za/2009/05/03/poisson-disk-sampling/

[anyweez]

Added in 7808ad7

Not using Poisson disk sampling yet (using random probabilities instead). Looks decent but I'm going to break off a separate ticket to experiment with other methods that may make distributions a little more interesting. Random's a bit boring.