Convert turfs to use add_overlays(), eliminating the turf_overlay_holder!

- Converted as much as I could find about turf overlays to use add_overlay().
- This should be enough to stop BYOND from crashing, so we can eliminate the turf_overlay_holder hack.
- This also lets us remove the anti-corruption hacks from walls and open space.
- ZAS gas overlays can use priority overlays, so this also fixes the gas-goes-away-when-crowbarring-plating issue.
- Stuff like that

code/_helpers/unsorted.dm

@@ -800,7 +800,6 @@ proc/GaussRandRound(var/sigma,var/roundto)
 						var/old_dir1 = T.dir
 						var/old_icon_state1 = T.icon_state
 						var/old_icon1 = T.icon
-						var/old_overlays = T.overlays.Copy()
 						var/old_underlays = T.underlays.Copy()
 						var/old_decals = T.decals ? T.decals.Copy() : null
 
@@ -808,11 +807,9 @@ proc/GaussRandRound(var/sigma,var/roundto)
 						X.set_dir(old_dir1)
 						X.icon_state = old_icon_state1
 						X.icon = old_icon1
-						X.overlays = old_overlays
+						X.copy_overlays(T, TRUE)
 						X.underlays = old_underlays
 						X.decals = old_decals
-						if(old_decals)
-							X.apply_decals()
 
 					//Move the air from source to dest
 					var/turf/simulated/ST = T
@@ -838,14 +835,10 @@ proc/GaussRandRound(var/sigma,var/roundto)
 					if(shuttlework)
 						var/turf/simulated/shuttle/SS = T
 						SS.landed_holder.leave_turf()
-
 					else if(turftoleave)
 						T.ChangeTurf(turftoleave)
-						T.apply_decals()
-
 					else
 						T.ChangeTurf(get_base_turf_by_area(T))
-						T.apply_decals()
 
 					refined_src -= T
 					refined_trg -= B