Reduce cave density

[olls]

There are far too many caves. We can't reduce their density by tweaking the cave_chance attribute, because that results in no caves being generated because the CA requires many air_points to be touching for a cave to be generated.

Maybe some kind of generating the air_points uniformly across the map, we generate seed points uniformly at a much lower density, then generate air_points around those seed points with a high density.