check whether the map's submap grid

is somehow corrupted,
before adding a spawn point to it.
fixed segfault on flooding strange temple room
Let webs be woven over slime.

Both web weaver and slimy are mutations that can coexist.
making undergound NPCs hog less CPU

NPCs no longer flail about looking for various shops.
Functionally, they still do the same thing. Just much faster.