Improve the flowable material system

[emezeske]

Right now the flowing algorithm is really basic, and while it does an okay job sometimes, there are a lot of things that it doesn't handle very well. In particular, a couple of problems I've come across are:

• A vertical wall of water, when flowed, just spreads from its bottom layer, instead of pouring out in a slope. (Picture a dam bursting -- the water would only come out the bottom of the dam, and the water above that would just stay still.)
• There's no concept of a "source." Thus, if you have a water canal, and change its direction, the water that was flowing in the old direction will never go away.