Liquid transformation refactoring

[flyx]

Rewrote the whole transformLiquids algorithm. Major changes:

• The MapNode of flowing liquids now stores a bit for "is this liquid flowing down" additionally to the liquid level (which uses 3 bits). This makes it easier to determine which nodes the liquid flows into.
• One iteration does not change any MapNode apart from the one it operates on. This makes future changes to the algorithm easier as the flowing logic is only implemented once (the former code implemented it twice - for the node the liquid flows from, and for the node the liquid flows into)
• transformLiquids now also operates on air nodes so they can be turned into flowing liquids if neccessary (was formerly done by the code operating on the neighboring liquid)
• ONLY CHANGE FOR THE END-USER: If a flowing liquid hasn't any solid neighbor node, its level decreases faster (by 1 more). This prevents liquids from spreading too wide in open terrains.

[erlehmann]

I have no use for the minetest-mac build system and the merge commit. I did cherry-pick the other commits. Please provide clean pull requests in the future. Also, think about rebasing your feature branches before sending a pull request.