Fixes topdown emissives/emissive blockers and blood trail rendering (#92430)

## About The Pull Request

Fixes this

<img width="983" height="591" alt="zen_Mhqh2OqiU4"
src="https://github.com/user-attachments/assets/b3275052-1b24-404b-82bc-a4c8e88bdbcf"
/>

This code is mildly bad, but this is the best way we can fix FLOAT_LAYER
topdown emissives/blockers rendering ontop of everything else.

Also added logging/unit testing for blood trails spawned outside of
holders, mappers can use holders to spawn trails (which is the right way
to add them to your maps)

## Changelog
🆑
fix: Fixed blood/glass floor glow going over objects
/🆑

(cherry picked from commit ea0fa299c4)

code/__DEFINES/layers.dm

@@ -153,6 +153,9 @@
 // NOTICE: we break from the pattern of increasing in steps of like 0.01 here
 // Because TOPDOWN_LAYER is 10000 and that's enough to floating point our modifications away
 
+// Must be equal to the offset of the highest topdown layer
+#define TOPDOWN_LAYER_COUNT 18
+
 //lower than LOW_FLOOR_LAYER, for turfs with stuff on the edge that should be covered by other turfs
 #define LOWER_FLOOR_LAYER (1 + TOPDOWN_LAYER)
 #define LOW_FLOOR_LAYER (2 + TOPDOWN_LAYER)
@@ -172,7 +175,7 @@
 #define ABOVE_OPEN_TURF_LAYER (15 + TOPDOWN_LAYER)
 #define LOWER_RUNE_LAYER (16 + TOPDOWN_LAYER)
 #define RUNE_LAYER (17 + TOPDOWN_LAYER)
-#define CLEANABLE_FLOOR_OBJECT_LAYER (21 + TOPDOWN_LAYER)
+#define CLEANABLE_FLOOR_OBJECT_LAYER (18 + TOPDOWN_LAYER)
 
 //Placeholders in case the game plane and possibly other things between it and the floor plane are ever made into topdown planes
 
@@ -188,6 +191,8 @@
 // GAME_PLANE layers
 #define BULLET_HOLE_LAYER 2.06
 #define ABOVE_NORMAL_TURF_LAYER 2.08
+#define FLOOR_EMISSIVE_START_LAYER 2.09
+#define FLOOR_EMISSIVE_END_LAYER 2.26
 #define GAS_PIPE_HIDDEN_LAYER 2.35 //layer = initial(layer) + piping_layer / 1000 in atmospherics/update_icon() to determine order of pipe overlap
 #define WIRE_BRIDGE_LAYER 2.44
 #define WIRE_TERMINAL_LAYER 2.45

code/__HELPERS/lighting.dm

@@ -1,5 +1,13 @@
+#define TOPDOWN_TO_EMISSIVE_LAYER(layer) LERP(FLOOR_EMISSIVE_START_LAYER, FLOOR_EMISSIVE_END_LAYER, (layer - (TOPDOWN_LAYER + 1)) / TOPDOWN_LAYER_COUNT)
+
 /// Produces a mutable appearance glued to the [EMISSIVE_PLANE] dyed to be the [EMISSIVE_COLOR].
-/proc/emissive_appearance(icon, icon_state = "", atom/offset_spokesman, layer = FLOAT_LAYER, alpha = 255, appearance_flags = NONE, offset_const, apply_bloom = TRUE)
+/proc/emissive_appearance(icon, icon_state = "", atom/offset_spokesman, layer, alpha = 255, appearance_flags = NONE, offset_const, apply_bloom = TRUE)
+	if (isnull(layer))
+		if(IS_TOPDOWN_PLANE(offset_spokesman.plane))
+			layer = TOPDOWN_TO_EMISSIVE_LAYER(offset_spokesman.layer)
+		else
+			layer = FLOAT_LAYER
+
 	var/mutable_appearance/appearance = mutable_appearance(icon, icon_state, layer, offset_spokesman, EMISSIVE_PLANE, 255, appearance_flags | EMISSIVE_APPEARANCE_FLAGS, offset_const)
 	if(alpha == 255)
 		if (apply_bloom)
@@ -27,6 +35,10 @@
 	blocker.icon = make_blocker.icon
 	blocker.icon_state = make_blocker.icon_state
 	// blocker.layer = FLOAT_LAYER // Implied, FLOAT_LAYER is default for appearances
+	// If we keep this on a FLOAT_LAYER on a topdown object it'll render ontop of everything
+	// So we need to force it to render at a saner layer
+	if(IS_TOPDOWN_PLANE(make_blocker.plane))
+		blocker.layer = TOPDOWN_TO_EMISSIVE_LAYER(make_blocker.layer)
 	blocker.appearance_flags |= make_blocker.appearance_flags | EMISSIVE_APPEARANCE_FLAGS
 	blocker.dir = make_blocker.dir
 	if(make_blocker.alpha == 255)
@@ -41,7 +53,12 @@
 	return blocker
 
 /// Produces a mutable appearance glued to the [EMISSIVE_PLANE] dyed to be the [EM_BLOCK_COLOR].
-/proc/emissive_blocker(icon, icon_state = "", atom/offset_spokesman, layer = FLOAT_LAYER, alpha = 255, appearance_flags = NONE, offset_const)
+/proc/emissive_blocker(icon, icon_state = "", atom/offset_spokesman, layer, alpha = 255, appearance_flags = NONE, offset_const)
+	if (isnull(layer))
+		if(IS_TOPDOWN_PLANE(offset_spokesman.plane))
+			layer = TOPDOWN_TO_EMISSIVE_LAYER(offset_spokesman.layer)
+		else
+			layer = FLOAT_LAYER
 	var/mutable_appearance/appearance = mutable_appearance(icon, icon_state, layer, offset_spokesman, EMISSIVE_PLANE, alpha, appearance_flags | EMISSIVE_APPEARANCE_FLAGS, offset_const)
 	if(alpha == 255)
 		appearance.color = GLOB.em_block_color
@@ -77,3 +94,5 @@
 	var/atom/movable/vis_cast = make_blocker
 	vis_cast.vis_contents += hand_back
 	return hand_back
+
+#undef TOPDOWN_TO_EMISSIVE_LAYER

code/game/objects/effects/decals/cleanable/blood.dm

@@ -119,7 +119,7 @@
 		. += blood_emissive(icon, icon_state)
 
 /obj/effect/decal/cleanable/blood/proc/blood_emissive(icon_to_use, icon_state_to_use)
-	return emissive_appearance(icon_to_use, icon_state_to_use, src, layer, 255 * emissive_alpha / alpha)
+	return emissive_appearance(icon_to_use, icon_state_to_use, src, alpha = 255 * emissive_alpha / alpha)
 
 /obj/effect/decal/cleanable/blood/lazy_init_reagents()
 	if (reagents)
@@ -459,6 +459,25 @@
 	/// Beyond a threshold we change to a bloodier icon state
 	var/very_bloody = FALSE
 
+/obj/effect/decal/cleanable/blood/trail/Initialize(mapload, list/datum/disease/diseases, list/blood_or_dna)
+	. = ..()
+	// Despite having VIS_INHERIT_PLANE, our emissives still inherit our plane offset, so we need to inherit our parent's offset to have them render correctly
+	if(istype(loc, /obj/effect/decal/cleanable/blood/trail_holder))
+		SET_PLANE_EXPLICIT(src, initial(plane), loc)
+		if (emissive_alpha && !dried)
+			update_appearance() // correct our emissive
+		return
+
+
+#ifndef UNIT_TESTS
+	if (mapload)
+		log_mapping("[src] spawned outside of a trail holder at [AREACOORD(src)]!")
+		return INITIALIZE_HINT_QDEL
+#endif
+
+	stack_trace("[src] spawned outside of a trail holder at [AREACOORD(src)]!")
+	return INITIALIZE_HINT_QDEL
+
 /obj/effect/decal/cleanable/blood/trail/update_desc(updates)
 	. = ..()
 	desc = "A [dried ? "dried " : ""]trail of [get_blood_string()]."

code/modules/unit_tests/unit_test.dm

@@ -276,8 +276,10 @@ GLOBAL_VAR_INIT(focused_tests, focused_tests())
 		/obj/effect/bug_moving,
 		//The abstract grown item expects a seed, but doesn't have one
 		/obj/item/food/grown,
-		///Single use case holder atom requiring a user
+		//Single use case holder atom requiring a user
 		/atom/movable/looking_holder,
+		//Should not exist outside of holders
+		/obj/effect/decal/cleanable/blood/trail,
 	)
 
 	// Everything that follows is a typesof() check.

code/modules/unit_tests/washing.dm

@@ -10,7 +10,7 @@
 	VAR_PRIVATE/clean_sig_caught = 0
 
 /datum/unit_test/washing/Run()
-	for(var/i in subtypesof(/obj/effect/decal/cleanable) + cleanable_bonus_list)
+	for(var/i in subtypesof(/obj/effect/decal/cleanable) + cleanable_bonus_list - uncreatables)
 		var/atom/movable/to_clean = allocate(i)
 		var/mopable = HAS_TRAIT(to_clean, TRAIT_MOPABLE)