changes:
Z-levels below the surface now use 'rocky' ash instead of ash turfs. (Sprites from TG)
Fixed an issue where openturfs would incorrectly inherit their mimiced turf's gender.
Tweaked the names & descriptions of sand (now ash) and rock.
Reduced some pointless locate() calls in icon smoothing code for turf paths.
Replaced the empty space map with runtime Z expansion.
Fixes#2756.