Cross-Z Lighting (#2281)

This PR edits the lighting engine's turf selection algorithm to also include turfs below openturf tiles, allowing for cross-Z lighting.

changes:

Lights now will now shine down Z-levels when they light up an open turf.
Commented-out openturf starlight pending making it not pummel SSlighting.
Openspace overlays are now only queued if they are not already in the queue.
Lighting overlays will now also update their associated openturf overlay on update if they have one.
Removed an old unused message from the asteroid generation subsystem.

code/modules/lighting/lighting_source.dm

@@ -371,6 +371,7 @@
 	var/Sy = source_turf.y
 
 	FOR_DVIEW(T, Ceiling(light_range), source_turf, 0)
+		check_t:
 		if (light_angle && check_light_cone(T.x, T.y))
 			continue
 
@@ -379,6 +380,11 @@
 			corners[C] = 0
 
 		turfs += T
+
+		if (istype(T, /turf/simulated/open) && T:below)
+			T = T:below	// Consider the turf below us as well. (Z-lights)
+			goto check_t
+
 	END_FOR_DVIEW
 
 	LAZYINITLIST(affecting_turfs)