Add some helpers for turf translations

code/_helpers/turfs.dm

@@ -104,6 +104,9 @@
 	//Are we doing shuttlework? Just to save another type check later.
 	var/shuttlework = 0
 
+	T.pre_translate_A(B)
+	B.pre_translate_B(T)
+
 	//Shuttle turfs handle their own fancy moving.
 	if(istype(T,/turf/simulated/shuttle))
 		shuttlework = 1
@@ -116,7 +119,6 @@
 		var/old_dir1 = T.dir
 		var/old_icon_state1 = T.icon_state
 		var/old_icon1 = T.icon
-		var/old_underlays = T.underlays.Copy()
 		var/old_decals = T.decals ? T.decals.Copy() : null
 
 		X = B.ChangeTurf(T.type)
@@ -124,7 +126,6 @@
 		X.icon_state = old_icon_state1
 		X.icon = old_icon1
 		X.copy_overlays(T, TRUE)
-		X.underlays = old_underlays
 		X.decals = old_decals
 
 	//Move the air from source to dest
@@ -164,6 +165,9 @@
 	else
 		T.ChangeTurf(get_base_turf_by_area(T))
 
+	T.post_translate_A(B)
+	B.post_translate_B(T)
+
 	return TRUE
 
 //Used for border objects. This returns true if this atom is on the border between the two specified turfs

code/game/turfs/turf.dm

@@ -394,3 +394,17 @@
 		ChangeTurf(/turf/simulated/floor/airless, preserve_outdoors = TRUE)
 		return TRUE
 	return FALSE
+
+
+// We're about to be the A-side in a turf translation
+/turf/proc/pre_translate_A(var/turf/B)
+	return
+// We're about to be the B-side in a turf translation
+/turf/proc/pre_translate_B(var/turf/A)
+	return
+// We were the the A-side in a turf translation
+/turf/proc/post_translate_A(var/turf/B)
+	return
+// We were the the B-side in a turf translation
+/turf/proc/post_translate_B(var/turf/A)
+	return