De-snowflake thrown objects hitting turfs.

* Port of https://github.com/Baystation12/Baystation12/pull/16942
* Actually tell turfs when a thrown object hits them, and let them decide what to do about it!
  * We do this by calling hitby(), which is how it already works for obj and mob, so this makes behavior consistent.
* This allows us to cleanly solve the problem of a thrown object landing on open space without falling.

code/game/atoms_movable.dm

@@ -115,12 +115,7 @@
 	else if(isturf(hit_atom))
 		src.throwing = 0
 		var/turf/T = hit_atom
-		if(T.density)
-			spawn(2)
-				step(src, turn(src.last_move, 180))
-			if(istype(src,/mob/living))
-				var/mob/living/M = src
-				M.turf_collision(T, speed)
+		T.hitby(src,speed)
 
 //decided whether a movable atom being thrown can pass through the turf it is in.
 /atom/movable/proc/hit_check(var/speed)

code/game/turfs/turf.dm

@@ -243,3 +243,11 @@ var/const/enterloopsanity = 100
 /turf/proc/update_blood_overlays()
 	return
 
+// Called when turf is hit by a thrown object
+/turf/hitby(atom/movable/AM as mob|obj, var/speed)
+	if(src.density)
+		spawn(2)
+			step(AM, turn(AM.last_move, 180))
+		if(isliving(AM))
+			var/mob/living/M = AM
+			M.turf_collision(src, speed)

code/modules/multiz/turf.dm

@@ -36,6 +36,11 @@
 	..()
 	mover.fall()
 
+// Called when thrown object lands on this turf.
+/turf/simulated/open/hitby(var/atom/movable/AM, var/speed)
+	. = ..()
+	AM.fall()
+
 /turf/simulated/open/proc/update()
 	below = GetBelow(src)
 	turf_changed_event.register(below, src, /turf/simulated/open/update_icon)