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/controllers/subsystems/floor_decals.dm

@@ -1,28 +0,0 @@
-//
-// Floor Decals Initialization Subsystem
-// This is part of the giant decal hack that works around a BYOND bug where DreamDaemon will crash if you
-// update overlays on turfs too much.
-// The master_controller on Polaris used to init decals prior to initializing areas (which initilized turfs)
-// Now that we switched to subsystems we still want to do the same thing, so this takes care of it.
-//
-SUBSYSTEM_DEF(floor_decals)
-	name = "Floor Decals"
-	init_order = INIT_ORDER_DECALS
-	flags = SS_NO_FIRE
-
-/datum/controller/subsystem/floor_decals/Initialize(timeofday)
-	if(floor_decals_initialized)
-		return ..()
-	to_world_log("Initializing Floor Decals")
-	admin_notice("<span class='danger'>Initializing Floor Decals</span>", R_DEBUG)
-	var/list/turfs_with_decals = list()
-	for(var/obj/effect/floor_decal/D in world)
-		var/T = D.add_to_turf_decals()
-		if(T) turfs_with_decals |= T
-		CHECK_TICK
-	for(var/item in turfs_with_decals)
-		var/turf/T = item
-		if(T.decals) T.apply_decals()
-		CHECK_TICK
-	floor_decals_initialized = TRUE
-	return ..()