Review GAT terrain types and their interpretation (incl. serverside/gameplay implications)

[rdw-software]

The original GAT specification included a more detailed interpretation of the terrain types (tile flags) than I am currently able to confirm myself. Since the client clearly doesn't interpret them as such, I'm removing those parts while adding this issue as a reminder that they're to be reviewed once I can more comfortably test the assumptions and verify the research others have done in terms of a more detailed interpretation of those flags.

Specifically, the whole "underwater vs. ground-level" differentiation is something that seems a little all over the place right now, and I haven't seen anything in the client that would care even one bit about water tiles.

I'd assume that the zone server cares much more about the different types than the client's basic "can I move there?" checks I've seen. For the time being, it's not really all that relevant to the client itself, though.