Per position water flow

[fire]

func _get_water_flow(positions):
	var ret = []
	return ret

To give rivers with turbulence and flow maps to the simulation's flow direction.

[fire]

Simulating from top to bottom is lacking. It'll be more visually accurate to also have flow simulation.

https://twitter.com/iFiery/status/1350905604932472833

[fire]

I don't have the original test cases. How do I know this works? https://github.com/godot-extended-libraries/hydro/tree/flow-vector-4