Cut down on z-level generation for shuttles (#13573)

code/modules/overmap/ships/landable.dm

@@ -7,6 +7,7 @@
 	var/obj/effect/shuttle_landmark/ship/landmark       // Record our open space landmark for easy reference.
 	var/multiz = 0										// Index of multi-z levels, starts at 0
 	var/status = SHIP_STATUS_LANDED
+	var/subshuttle = FALSE								// TRUE/FALSE: Is this a ship that starts docked to another ship?
 	icon_state = "shuttle"
 	moving_state = "shuttle_moving"
 
@@ -34,8 +35,12 @@
 
 // We autobuild our z levels.
 /obj/effect/overmap/visitable/ship/landable/find_z_levels()
-	for(var/i = 0 to multiz)
-		world.maxz++
+	if(isStationLevel(z) || multiz || subshuttle)
+		// If we're not on station and not multi-z or inside an existing ship, then we already had a z-level generated for us; let's use that
+		for(var/i = 0 to multiz)
+			world.maxz++
+			map_z += world.maxz
+	else
 		map_z += world.maxz
 
 	var/turf/center_loc = locate(round(world.maxx/2), round(world.maxy/2), world.maxz)
@@ -168,4 +173,4 @@
 		if(SHIP_STATUS_TRANSIT)
 			return "Maneuvering under secondary thrust."
 		if(SHIP_STATUS_OVERMAP)
-			return "In open space."
+			return "In open space."