Quite a large update changing the rivers generation.
- Visible river width depends on the streamflow (discharge) of water flowing in it.
- All geographical features data are now in EPSG:3785 projection (Mercator).
- In addition to rivers as multipoligons in rivers shapefile, there are also rivers as 3D-multilinestrings with z coordinate in riversz shapefile. The latter is a schematic representation of the river system trees.
New example planet added. Also corrected and expanded the tutorial on making a map in the equirectangular projection.