major changes to river generation:
- every landmass has a river if possible, even small islands
- there are two map options controlling rivers: River Length / Number of Rivers, and River Forks
- the amount of river tiles for each landmass is determined by how much rainfall falls on that landmass in total (rainfall being a quantification of the terrain types and features)
- rivers are in general shorter, more numerous, and with fewer tributaries
- rivers connecting lakes may have been broken in the previous version, and now it's back
other changes
- finally, floodplains tiles actually work
- oases now only spawn in empty desert tiles surrounded entirely by empty desert tiles
