Add a surface water system

[alice-i-cecile]

Essential for making the world come alive.

Initial gameplay elements:

• rivers
• lakes
• weather
• bridge structure

Initial gameplay effects:

• water flows downhill
• water pools
• water evaporates over time unless replenished
• units cannot cross water
• structures do not work underwater
• most organisms die if underwater for too long
• water must be used to grow plants

[alice-i-cecile]

We'd like a friendly toon-shaded effect, as seen in this tutorial: https://roystan.net/articles/toon-water/

Another shadertoy example: https://www.shadertoy.com/view/4tffzN