------Swamps------
— Textures from: gimp, gamiano.de
— Sounds from: http://www.freesound.org/people/dobroide/sounds/16771/, supertux (GPL)
— I looked at snow mod for mapgen and
— at the jungletree mod for birches.

TODO:
— fix swampwater
— add CC0 16px textures for the birch
— maybe add some swamp huts