Asteroid tweaks (#3951)

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.

code/controllers/subsystems/initialization/atlas.dm

@@ -63,6 +63,8 @@ var/datum/controller/subsystem/atlas/SSatlas
 
 	setup_multiz()
 
+	current_map.finalize_load()
+
 	QDEL_NULL(maploader)
 
 	..()