There were sometimes missing water patches near a lake's edge, when the neighbour catchment area was lower.
This commit allows to divide a cell into several mini-catchment basins, to fill only a part of it with water.
makes slopes more irregular and natural-looking, allows overhanging.
This is done by generating an intermediate 2D elevation map and, for each node in 3D, add a 2D offset vector to the position, and seek this position on the heightmap.
