Heretic Influences use alt appearances (so ghosts can now see them (semi-transparent)) (#85932)

## About The Pull Request

Removes the rather overcomplicated system behind heretic influences and
makes them use alt appearances

Fixes a few bugs involving alt appearances that I noticed in making them

I made this PR 7 months ago and forgot to Pr it

## Why It's Good For The Game

Less complex codes and lets observers get a cut in on the action.

Should fix #77530

## Changelog

🆑 Melbert
refactor: Refactored heretic influences a tiny bit, now ghosts can see
them! Report any oddities.
/🆑

code/modules/mob/dead/observer/observer.dm

@@ -141,11 +141,8 @@ GLOBAL_VAR_INIT(observer_default_invisibility, INVISIBILITY_OBSERVER)
 
 	add_to_dead_mob_list()
 
-	for(var/v in GLOB.active_alternate_appearances)
-		if(!v)
-			continue
-		var/datum/atom_hud/alternate_appearance/AA = v
-		AA.onNewMob(src)
+	for(var/datum/atom_hud/alternate_appearance/alt_hud as anything in GLOB.active_alternate_appearances)
+		alt_hud.apply_to_new_mob(src)
 
 	. = ..()