Blob Maintenance: Stairs are Hard (#9146)

* Fix: Blobs respect gravity if not otherwise prevented.
Tweak: Blobs do not directly interact with stairs
Tweak: Blobs, when expanding against a stair's middle structure, can grow up Z-levels if the turf allows.
Tweak: Blobs, if controlled by a player, may grow upward or downward, if the turfs allow.
Fix: Blob Overmind is no longer affected by gravity.

* Fringe case deleted blobs are null-loc'd for GC so they are not stuck in-map.
Railing checks properly function.

code/game/objects/structures/railing.dm

@@ -64,6 +64,15 @@
 		new /obj/item/stack/rods(get_turf(src))
 		qdel(src)
 
+/obj/structure/railing/blob_act(var/obj/structure/blob/B)
+	. = ..()
+
+	if(B?.overmind?.blob_type)
+		take_damage(rand(B.overmind.blob_type.damage_lower, B.overmind.blob_type.damage_upper))
+		return
+
+	take_damage(rand(10,30))
+
 /obj/structure/railing/proc/NeighborsCheck(var/UpdateNeighbors = 1)
 	check = 0
 	//if (!anchored) return