fix gas overlays persisting after rebuilds

slight visual artifact when zones are rebuilt, overlays will be
missing until the end of the current tick.  otherwise works fine.

Signed-off-by: Mloc-Argent <colmohici@gmail.com>

code/ZAS/Zone.dm

@@ -118,6 +118,7 @@ Class Procs:
 	if(invalid) return //Short circuit for explosions where rebuild is called many times over.
 	c_invalidate()
 	for(var/turf/simulated/T in contents)
+		T.update_graphic(graphic_remove = air.graphic) //we need to remove the overlays so they're not doubled when the zone is rebuilt
 		//T.dbg(invalid_zone)
 		T.needs_air_update = 0 //Reset the marker so that it will be added to the list.
 		air_master.mark_for_update(T)