[MIRROR] Fixes an issue with multiz visholders duplicating [MDB IGNORE] (#19727)

* Fixes an issue with multiz visholders duplicating (#73835)

## About The Pull Request

I think the inherent issue here has to do with turf refs or something,
but the big problem was I assumed if there was only once source, then
the turf must be new. This was an invalid assumption, since we can add
sources more then once, and | is used

Because of this, each time a shuttle moved, we'd add an extra source for
no reason, and stack extra vis holders. This lead to really bad
clientside lag on some out of repo maps, which is how this issue came to
my attention.

Instead checking if the sources list exists or not solves the problem
pretty handily, so let's do that.

## Why It's Good For The Game

Closes #73834, prevents a potential future OOM

* Fixes an issue with multiz visholders duplicating

---------

Co-authored-by: LemonInTheDark <58055496+LemonInTheDark@users.noreply.github.com>

code/datums/elements/turf_transparency.dm

@@ -96,12 +96,9 @@ GLOBAL_LIST_EMPTY(pillars_by_z)
 /// Displays a turf from the z level below us on our level
 /datum/z_pillar/proc/display_turf(turf/to_display, turf/source)
 	var/list/sources = turf_sources[to_display]
-	if(!sources)
-		sources = list()
-		turf_sources[to_display] = sources
-	sources |= source
 
-	if(length(sources) != 1) // If we aren't the first to request this turf, return
+	if(sources) // If we aren't the first to request this turf, return
+		sources |= source
 		var/obj/effect/abstract/z_holder/holding = drawing_object[to_display]
 		if(!holding)
 			return
@@ -118,6 +115,11 @@ GLOBAL_LIST_EMPTY(pillars_by_z)
 		visual_target.vis_contents += to_display
 		return
 
+	// Otherwise, we need to create a new set of sources. let's do that yeah?
+	sources = list()
+	turf_sources[to_display] = sources
+	sources |= source
+
 	var/turf/visual_target = to_display.above()
 	if(istransparentturf(visual_target) || isopenspaceturf(visual_target))
 		visual_target.vis_contents += to_display