Add roofs (#2737)

Implements roofs

code/modules/mining/mine_turfs.dm

@@ -516,6 +516,8 @@
 	has_resources = 1
 	footstep_sound = "gravelstep"
 
+	roof_type = null
+
 // Copypaste parent for performance.
 /turf/simulated/floor/asteroid/Initialize()
 	if(initialized)
@@ -764,8 +766,7 @@
 			var/area/below_area = below.loc		// Let's just assume that the turf is not in nullspace.
 			if(below_area.station_area)
 				user << "<span class='alert'>You strike metal!</span>"
-				var/turf/T = ChangeTurf(/turf/simulated/floor/airless)
-				T.icon_state = "asteroidplating"
+				below.spawn_roof(ROOF_FORCE_SPAWN)
 			else
 				ChangeTurf(/turf/space)