[MIRROR] Undertile Element Logic Refactor, or Catwalks Aren't Affected by Ambient Occlusion Anymore [MDB IGNORE] (#17845)

* Undertile Element Logic Refactor, or Catwalks Aren't Affected by Ambient Occlusion Anymore (#71555)

## About The Pull Request
It was bugging me how catwalks would just be stuck rendering on the game
plane in order to be above the pipes and all the other underfloor
objects, because it meant that they stood out due to being affected by
ambient occlusion.

So I decided to change that, and the best change I could come up with,
was to refactor the logic of `/datum/element/undertile` in order to
actually allow us to do exactly what we wanted by having three different
states of underfloor visibility, which in turn allowed me to slap
everything that wasn't accessible on the floor plane rather than
whatever plane they were on, effectively making it so catwalk tiles
wouldn't need to be on the game plane anymore. :)

Also fixes https://github.com/tgstation/tgstation/issues/63590 while I'm
at it :)

## Why It's Good For The Game
Seeing ambient occlusion on catwalks make them stand out in a jarring
way, now that won't be the case anymore!

Now, instead, you get something like this, which _absolutely_ looks like
it fits in!

![image](https://user-images.githubusercontent.com/58045821/204106823-95b77a6b-b9c1-4494-b2f8-3b586c42428c.png)

## Changelog

🆑 GoldenAlpharex
refactor: Refactored the way the undertile component works, to allow it
to have a bit more granularity as to when it's meant to be covered, but
still visible, like for catwalks!
fix: Catwalks no longer are affected by ambient occlusion, and now
properly feel like actual floor tiles.
/🆑

* Undertile Element Logic Refactor, or Catwalks Aren't Affected by Ambient Occlusion Anymore

Co-authored-by: GoldenAlpharex <58045821+GoldenAlpharex@users.noreply.github.com>

code/datums/elements/undertile.dm

@@ -19,6 +19,7 @@
 
 /datum/element/undertile/Attach(datum/target, invisibility_trait, invisibility_level = INVISIBILITY_MAXIMUM, tile_overlay, use_alpha = TRUE, use_anchor = FALSE)
 	. = ..()
+
 	if(!ismovable(target))
 		return ELEMENT_INCOMPATIBLE
 
@@ -30,37 +31,51 @@
 	src.use_alpha = use_alpha
 	src.use_anchor = use_anchor
 
+
 ///called when a tile has been covered or uncovered
-/datum/element/undertile/proc/hide(atom/movable/source, covered)
+/datum/element/undertile/proc/hide(atom/movable/source, underfloor_accessibility)
 	SIGNAL_HANDLER
 
-	source.invisibility = covered ? invisibility_level : 0
+	source.invisibility = underfloor_accessibility < UNDERFLOOR_VISIBLE ? invisibility_level : 0
 
 	var/turf/T = get_turf(source)
 
-	if(covered)
-		if(invisibility_trait)
-			ADD_TRAIT(source, invisibility_trait, ELEMENT_TRAIT(type))
+	if(underfloor_accessibility < UNDERFLOOR_INTERACTABLE)
+		SET_PLANE_IMPLICIT(source, FLOOR_PLANE) // We do this so that turfs that allow you to see what's underneath them don't have to be on the game plane (which causes ambient occlusion weirdness)
+
 		if(tile_overlay)
 			T.add_overlay(tile_overlay)
-		if(use_alpha)
-			source.alpha = ALPHA_UNDERTILE
+
 		if(use_anchor)
 			source.set_anchored(TRUE)
 
+		if(underfloor_accessibility < UNDERFLOOR_VISIBLE)
+			if(use_alpha)
+				source.alpha = ALPHA_UNDERTILE
+
+			if(invisibility_trait)
+				ADD_TRAIT(source, invisibility_trait, ELEMENT_TRAIT(type))
+
 	else
+		SET_PLANE_IMPLICIT(source, initial(source.plane))
+
 		if(invisibility_trait)
 			REMOVE_TRAIT(source, invisibility_trait, ELEMENT_TRAIT(type))
+
 		if(tile_overlay)
 			T.overlays -= tile_overlay
+
 		if(use_alpha)
-			source.alpha = 255
+			source.alpha = initial(source.alpha)
+
 		if(use_anchor)
 			source.set_anchored(FALSE)
 
-/datum/element/undertile/Detach(atom/movable/AM, visibility_trait, invisibility_level = INVISIBILITY_MAXIMUM)
+
+/datum/element/undertile/Detach(atom/movable/source, visibility_trait, invisibility_level = INVISIBILITY_MAXIMUM)
 	. = ..()
 
-	hide(AM, FALSE)
+	hide(source, UNDERFLOOR_INTERACTABLE)
+
 
 #undef ALPHA_UNDERTILE