Enables overmap on SC, adds Sif to SC skybox (#6954)

* Enables overmap on SC, adds wip Sif to SC skybox

* codersprites

code/modules/overmap/sectors.dm

@@ -6,7 +6,7 @@
 	scannable = TRUE
 	scanner_desc = "!! No Data Available !!"
 
-	var/list/map_z = list()
+	var/list/map_z = null
 	var/list/extra_z_levels //if you need to manually insist that these z-levels are part of this sector, for things like edge-of-map step trigger transitions rather than multi-z complexes
 
 	var/list/initial_generic_waypoints //store landmark_tag of landmarks that should be added to the actual lists below on init.
@@ -31,7 +31,8 @@
 	if(. == INITIALIZE_HINT_QDEL)
 		return
 
-	find_z_levels()     // This populates map_z and assigns z levels to the ship.
+	if(!map_z)			// If map_z is already defined, we don't need to find where we are
+		find_z_levels() // This populates map_z and assigns z levels to the ship.
 	register_z_levels() // This makes external calls to update global z level information.
 
 	if(!global.using_map.overmap_z)