Completely rewrote the liquid transformation. The algorithm now exclusively modifies the current node in one iteration. Another change is that MapNode.param2 now stores a bit that tells other nodes if a flowing liquid node flows downwards. This is accomplished by two masks on param2 for the "flow down" bit and the "liquid level" bits.

This will be the base of future improvements to the liquid flow. However, in the current state flowing liquid does not always disappear when cut off its source. Be aware that this branch is work in progress.