Resolves mutant species keeping old organs they shouldn't have (#73728)

## About The Pull Request

regenerate organs never removed invalid organs when regenerating
## Why It's Good For The Game

Fixes https://github.com/tgstation/tgstation/issues/73640
adds a unit test to prevent regressions in the future

## Changelog
🆑
fix: Plasmamen don't have hearts, again
/🆑

code/datums/bodypart_overlays/mutant_bodypart_overlay.dm

@@ -13,6 +13,10 @@
 	///Take on the dna/preference from whoever we're gonna be inserted in
 	var/imprint_on_next_insertion = TRUE
 
+/datum/bodypart_overlay/mutant/get_overlay(layer, obj/item/bodypart/limb)
+	inherit_color(limb) // If draw_color is not set yet, go ahead and do that
+	return ..()
+
 ///Completely random image and color generation (obeys what a player can choose from)
 /datum/bodypart_overlay/mutant/proc/randomize_appearance()
 	randomize_sprite()
@@ -90,8 +94,13 @@
 
 ///Give the organ its color. Force will override the existing one.
 /datum/bodypart_overlay/mutant/proc/inherit_color(obj/item/bodypart/ownerlimb, force)
+	if(isnull(ownerlimb))
+		draw_color = null
+		return TRUE
+
 	if(draw_color && !force)
-		return
+		return FALSE
+
 	switch(color_source)
 		if(ORGAN_COLOR_OVERRIDE)
 			draw_color = override_color(ownerlimb.draw_color)
@@ -102,6 +111,7 @@
 				return
 			var/mob/living/carbon/human/human_owner = ownerlimb.owner
 			draw_color = human_owner.hair_color
+
 	return TRUE
 
 ///Sprite accessories are singletons, stored list("Big Snout" = instance of /datum/sprite_accessory/snout/big), so here we get that singleton

code/modules/mob/living/carbon/human/species.dm

@@ -167,9 +167,9 @@ GLOBAL_LIST_EMPTY(features_by_species)
 	///What anim to use for gibbing
 	var/gib_anim = "gibbed-h"
 
-
-	//Do NOT remove by setting to null. use OR make an ASSOCIATED TRAIT.
-	//why does it work this way? because traits also disable the downsides of not having an organ, removing organs but not having the trait will make your species die
+	// Prefer anything other than setting these to null, such as TRAITS
+	// why?
+	// because traits also disable the downsides of not having an organ, removing organs but not having the trait or logic will make your species die
 
 	///Replaces default brain with a different organ
 	var/obj/item/organ/internal/brain/mutantbrain = /obj/item/organ/internal/brain
@@ -345,6 +345,9 @@ GLOBAL_LIST_EMPTY(features_by_species)
 		var/obj/item/organ/oldorgan = C.getorganslot(slot) //used in removing
 		var/obj/item/organ/neworgan = slot_mutantorgans[slot] //used in adding
 		if(!neworgan) //these can be null, if so we shouldn't regenerate
+			if(oldorgan) // although we also need to remove the old organ if it exists
+				oldorgan.Remove(C, special=TRUE)
+				qdel(oldorgan)
 			continue
 
 		if(visual_only && !initial(neworgan.visual))

code/modules/surgery/organs/external/_external_organs.dm

@@ -158,7 +158,7 @@
 	//Build the mob sprite and use it as our overlay
 	for(var/external_layer in bodypart_overlay.all_layers)
 		if(bodypart_overlay.layers & external_layer)
-			. += bodypart_overlay.get_overlay(external_layer, limb = null)
+			. += bodypart_overlay.get_overlay(external_layer, ownerlimb)
 
 ///The horns of a lizard!
 /obj/item/organ/external/horns
@@ -356,8 +356,11 @@
 	if(draw_layer != bitflag_to_layer(color_swapped_layer))
 		return ..()
 
-	var/list/rgb_list = rgb2num(draw_color)
-	overlay.color = rgb(color_inverse_base - rgb_list[1], color_inverse_base - rgb_list[2], color_inverse_base - rgb_list[3]) //inversa da color
+	if(draw_color) // can someone explain to me why draw_color is allowed to EVER BE AN EMPTY STRING
+		var/list/rgb_list = rgb2num(draw_color)
+		overlay.color = rgb(color_inverse_base - rgb_list[1], color_inverse_base - rgb_list[2], color_inverse_base - rgb_list[3]) //inversa da color
+	else
+		overlay.color = null
 
 /datum/bodypart_overlay/mutant/pod_hair/can_draw_on_bodypart(mob/living/carbon/human/human)
 	if((human.head?.flags_inv & HIDEHAIR) || (human.wear_mask?.flags_inv & HIDEHAIR))

code/modules/unit_tests/_unit_tests.dm

@@ -152,6 +152,7 @@
 #include "monkey_business.dm"
 #include "mouse_bite_cable.dm"
 #include "mutant_hands_consistency.dm"
+#include "mutant_organs.dm"
 #include "novaflower_burn.dm"
 #include "ntnetwork_tests.dm"
 #include "nuke_cinematic.dm"

code/modules/unit_tests/mutant_organs.dm

@@ -0,0 +1,65 @@
+/**
+ * Iterates over all species to ensure that organs are valid after being set to a mutant species
+ */
+/datum/unit_test/mutant_organs
+
+/datum/unit_test/mutant_organs/Run()
+	var/mob/living/carbon/human/consistent/dummy = allocate(/mob/living/carbon/human/consistent)
+	var/list/ignore = list(
+		/datum/species/dullahan,
+	)
+	var/list/species = subtypesof(/datum/species) - ignore
+	var/static/list/organs_we_care_about = list(
+		ORGAN_SLOT_BRAIN,
+		ORGAN_SLOT_HEART,
+		ORGAN_SLOT_LUNGS,
+		ORGAN_SLOT_EYES,
+		ORGAN_SLOT_EARS,
+		ORGAN_SLOT_TONGUE,
+		ORGAN_SLOT_LIVER,
+		ORGAN_SLOT_STOMACH,
+		ORGAN_SLOT_APPENDIX,
+	)
+
+	for(var/datum/species/species_type as anything in species)
+		// change them to the species
+		dummy.set_species(species_type)
+
+		// check all their organs
+		for(var/organ_slot in organs_we_care_about)
+			var/expected_type = slot_to_species_organ_type(organ_slot, species_type)
+			var/obj/item/organ/actual_organ = dummy.getorganslot(organ_slot)
+			if(isnull(actual_organ))
+				if(!isnull(expected_type))
+					TEST_FAIL("[species_type] did not update their [organ_slot] organ to [expected_type], no organ was found")
+					continue
+			else
+				if(isnull(expected_type))
+					TEST_FAIL("[species_type] did not remove their [organ_slot] organ")
+					continue
+
+				if(actual_organ.type != expected_type)
+					TEST_FAIL("[species_type] did not update their [organ_slot] organ to [expected_type], instead it was [actual_organ.type]")
+					continue
+
+/datum/unit_test/mutant_organs/proc/slot_to_species_organ_type(slot, datum/species/species)
+	switch(slot)
+		if(ORGAN_SLOT_BRAIN)
+			return initial(species.mutantbrain)
+		if(ORGAN_SLOT_HEART)
+			return initial(species.mutantheart)
+		if(ORGAN_SLOT_LUNGS)
+			return initial(species.mutantlungs)
+		if(ORGAN_SLOT_EYES)
+			return initial(species.mutanteyes)
+		if(ORGAN_SLOT_EARS)
+			return initial(species.mutantears)
+		if(ORGAN_SLOT_TONGUE)
+			return initial(species.mutanttongue)
+		if(ORGAN_SLOT_LIVER)
+			return initial(species.mutantliver)
+		if(ORGAN_SLOT_STOMACH)
+			return initial(species.mutantstomach)
+		if(ORGAN_SLOT_APPENDIX)
+			return initial(species.mutantappendix)
+	CRASH("Invalid organ slot [slot]") // just incase someone adds an organ we care about and forgets to add it here