Fixed multiple flag generation errors and bugs. i.e. Liquids and Topsoil

I fixed the (lack-of) liquid flag generation and made it work with modded/third-party liquids.

I also fixed the world-height bug with flags, when generated too high, would cause an exception.
Though I like the "FluidRegistry.lookupFluidForBlock(block) != null", I'm not entirely sure if we should put it in a repo where I haven't discussed Liquids at all yet. So this might look like magic for people. On the other hand, doing research is part of the job.