Isolated pools of deep water can generate

[tmewett]

I speculate that this is because deep water is a T_PATHING_BLOCKER, and only passable cells are considered for the connectivity checks during dungeon generation - but I'm not sure. It would be desirable to change this so that deep water tiles (or indeed cells of any tile?) cannot be disconnected from the level yet still count as a pathing blocker, so that features like #310 don't need to special-case it.

[ghost]

Disconnected lakes are quite frequent: about 12% of dungeons have one.

Here are some examples (master branch), all chosen because the isolated pool is on level 2 with a chasm above it on level 1.

• seed 5174 (the pool is surrounded not just by walls, but also by a chasm and a statue)
• seed 10736
• seed 15773
• seed 16536
• seed 22338 (tricky one: lake connects to a room accessible through a secret door, but the room's floor is unstable)
• seed 26367
• seed 26627 (disconnected pool is only 1 tile in size)
• seed 30142