Add support for Level Correction

[Feuerswut]

So i have a simple issue:
In real life, continental rivers flow downward, as they do.
In CS2, water is a wonky mess and there is no support for rivers that start at map edge X with height X and end at map edge Y with height Y < X.

E.g. the river source is broken.
So, the terrain tool should apply some level correction to rivers, such that there isn't a 40m height difference from world map edge to another.

Reference: Rhine, Konstanz (Bodensee) and before - Basel - Freiburg and after.

[nonta1234]

Water issues have been a significant concern since CS1. As for improvements, this idea is still in its early stages, but implementation will take a considerable amount of time due to the need for substantial modifications to the core code.