allow COMSIG_ATOM_UPDATE_OVERLAY responders to add managed overlays directly (#29091)

code/datums/elements/decal_element.dm

@@ -132,25 +132,10 @@
 		INVOKE_ASYNC(source, TYPE_PROC_REF(/obj/item/, update_slot_icon))
 	return ..()
 
-/datum/element/decal/proc/apply_overlay(atom/source)
+/datum/element/decal/proc/apply_overlay(atom/source, list/overlays)
 	SIGNAL_HANDLER  // COMSIG_ATOM_UPDATE_OVERLAYS
 
-	source.add_overlay(pic)
-	// TODO: Fix this disgusting hack
-	//
-	// `COMSIG_ATOM_UPDATE_OVERLAYS` is sent at the end of
-	// /atom/proc/update_icon's stanza for updating overlays, instead
-	// somewhere useful, like, during it. /tg/ handles this by sending
-	// a list of overlays with the signal, allowing receivers to add to
-	// the list, instead of returning their own.
-	//
-	// This is much saner and more flexible, but would require refactoring
-	// many many uses of update_overlay() across the code base, which is left
-	// as an exercise for the next poor sap to touch this code (probably me).
-	if(source.managed_overlays && !islist(source.managed_overlays))
-		source.managed_overlays = list(source.managed_overlays, pic)
-	else
-		LAZYDISTINCTADD(source.managed_overlays, pic)
+	overlays += pic
 
 /datum/element/decal/proc/clean_react(datum/source, clean_types)
 	SIGNAL_HANDLER  // COMSIG_COMPONENT_CLEAN_ACT

code/datums/elements/rust_element.dm

@@ -40,7 +40,7 @@
 	SIGNAL_HANDLER //COMSIG_ATOM_UPDATE_OVERLAYS
 
 	if(rust_overlay)
-		parent_atom.add_overlay(rust_overlay)
+		overlays += rust_overlay
 
 /// Because do_after sleeps we register the signal here and defer via an async call
 /datum/element/rust/proc/welder_tool_act(atom/source, obj/item/item, mob/user)