Humidity and temperature are now indexed to uniform altitude *over land
chunks* (and water chunks adjacent to land) rather than over the whole
range of altitude. This is necessary in order to satisfy the uniformity
conditions of the formula for weighted sum CDF.
Additionally, fixes the computation of whether a tree should be
generated or not. Previously, it was using a source of randomness
scaled to use much less than the full 0-1 range; this has been resolved.
This makes for much nicer and more gradual transitions between densities
and reduces the amount of completely barren landscapes, while also
making forests larger.
Finally, this commit adds a server command, debug_column, which returns
some useful debug information about a column given an x and y
coordinate. This is useful for debugging worldgen.
Also fix some things to use a consistent coordinate system and bump up
tree density and variation around tree density in hopes of creating more
aesthetically pleasing jungles. However, this does change worldgen a
bit, so please make sure you look through it before merging.
