Z-Lights Mk 2 (#4383)

changes:
Bidirectional source Z-lights have been reverted to single-direction in favor of corner z-bleed.
Z-mimic turfs will now average their light level with their mimiced turf to better approximate Z-lighting.
Openspaces have been made significantly less dark.
Corners no longer incorrectly always take the instant update pathway.
MultiZ helpers are now macros.
More things now properly respect area dynamic lighting settings.

code/modules/random_map/automata/caves.dm

@@ -1,6 +1,3 @@
-#define GET_BELOW_OR_NULL(atom, z) \
-	(!(z > world.maxz || z > 17 || z < 2) && z_levels & (1 << (z - 2))) ? get_step(atom, DOWN) : null
-
 /datum/random_map/automata/cave_system
 	iterations = 5
 	descriptor = "moon caves"
@@ -131,7 +128,7 @@
 			if(EMPTY_CHAR)
 				new_path = mineral_rich
 			if(FLOOR_CHAR)
-				var/turf/below = GET_BELOW_OR_NULL(T, T.z)
+				var/turf/below = GET_BELOW(T)
 				if(below)
 					var/area/below_area = below.loc		// Let's just assume that the turf is not in nullspace.
 					if(below_area.station_area)
@@ -164,5 +161,3 @@
 	target_turf_type = /turf/unsimulated/chasm_mask
 	mineral_sparse = /turf/simulated/floor/asteroid/ash
 	mineral_rich = /turf/simulated/floor/asteroid/ash
-
-#undef GET_BELOW_OR_NULL