Adds overmap sensors to the Southern Cross (#7436)

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

* codersprites

* Adds overmap sensors to the Southern Cross

code/modules/overmap/sectors.dm

@@ -5,7 +5,7 @@
 	name = "map object"
 	scannable = TRUE
 
-	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.
@@ -30,7 +30,8 @@
 	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)

code/modules/overmap/ships/computers/sensors.dm

@@ -6,6 +6,7 @@
 	circuit = /obj/item/weapon/circuitboard/sensors
 	extra_view = 4
 	var/obj/machinery/shipsensors/sensors
+	whitelisted_types = list(/obj/effect/overmap/visitable) // Stationary emplacements can support sensors
 
 /obj/machinery/computer/ship/sensors/attempt_hook_up(obj/effect/overmap/visitable/ship/sector)
 	if(!(. = ..()))

code/modules/overmap/ships/computers/ship.dm

@@ -7,10 +7,12 @@ somewhere on that shuttle. Subtypes of these can be then used to perform ship ov
 	var/obj/effect/overmap/visitable/ship/linked
 	var/list/viewers // Weakrefs to mobs in direct-view mode.
 	var/extra_view = 0 // how much the view is increased by when the mob is in overmap mode.
+	var/list/whitelisted_types = list(/obj/effect/overmap/visitable/ship)
+	var/list/blacklisted_types = list()
 
 // A late init operation called in SSshuttles, used to attach the thing to the right ship.
 /obj/machinery/computer/ship/proc/attempt_hook_up(obj/effect/overmap/visitable/ship/sector)
-	if(!istype(sector))
+	if(!sector || (sector.type in blacklisted_types) || !(sector.type in whitelisted_types))
 		return
 	if(sector.check_ownership(src))
 		linked = sector