Realistic water flow

[kateyy]

Ensure that water behaves "normal", also when not simulated by the physics.

It's probably hard to calculate large areas of water/fluids with the physics. Therefore we should add some routines that check that fluid terrain layers are never above the solids without being bound by them.

[kateyy]

An idea for this: We could define the current water level as particle drain, working as river or groundwater, so that it consumes all (water) particles without height/material changes. Another water level could represent smaller lakes, that will grow when water particles rest on them.