Add downstream migration behaviour of watersources

[zabka]

Hi! As I expanding my late game farming with irrigation, I had to set up first some small water behavior. Please find the related PR inspired by dynamic_liquids from FaceDeer. This small update makes water sources rolling downstream, scratching clay, breaking light structures made of sticks and finally filling rivers and ponds. Please note the water will never move at the same Y value, preventing endless loop.

The downside is I had to significantly push the frequency and chance of the related abm. While it makes lakes freezing at a steady pace and water flowing quickly to stable points, I don't have enough experience with Minetest to properly evaluate side effects and the excessive cpu usage. (NB: I generated a tsunami by planting hundreds of renewable water sources at the top of an hill yet the game slowed down but ultimately didn't crash)

My next steps will be to

• manage water scarcity based on abm optimization feedback
• checking the behavior of reservoirs made from waterproof materials
• checking the possibility of "pass thought" nodes (such as open/closed trapdoors) to manage waterflow
• checking the possibility of Y moving with conditions preventing infinite loop
• plant growth reliance on water supply

Thank you

[jeremyshannon]

Just a heads up, development has switched from github over to codeberg: https://codeberg.org/Mantar/Exile I can still merge from here if necessary, however.

[zabka]

Hi, I'm a git noob: if you can merge from here and it sync with codeberg, all right then. I ll check the doc on codeberg and send next PR directly to it.