Remove Direct Hit Limb Penalty against Xenos (#4788)

# About the pull request

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## Simple version

Selecting a body part on doll at bottom right applies an inherent
accuracy debuff to shots (an hand is harder to hit than chest).

This penalty still applied to sprite-click attacks on Xenomorphs,
despite the localized damage having no effect.

This means that you'd have to constantly reset your aimed part to chest
every time you try to shoot a Xeno, or would suffer a "free" penalty.

## Detailled version

Old flow went as follow:
 * Get base Hit Chance and Roll
* If it's only a near hit, or target's not the shot clicked target, the
shot targeted limb is reset to a random one
 * Hit chance receives a penalty based on aimed limb
 * We proceed checking hit chance vs roll

Because of the above problem this is changed to:

* Get base Hit Chance and Roll
* If it's a near hit (including limb penalty), a near xeno hit
(excluding limb penalty), or not the sprite-clicked target, aimed limb
is reset to a random one
* Else if the target is a xeno and it was a proper direct hit, there is
no penalty -- functionally similar to a chest shot
* Else Finally, apply per-limb penalty as usual
* We proceed checking hit chance vs roll

# Explain why it's good for the game
Avoids an invisible noobtrap debuff for just about everyone in the game
that isn't in the know - and the neccessity to constantly reset
targeting to chest for medics.

# Testing Photographs and Procedure
The problem was tested by a player on discord -- the fix i tested just
was spawning stuff and shooting up the place observing RNG misses. I did
not take detailed statistics over time to confirm.


# Changelog
:cl:
fix: Sprite-click shots onto Xenos are no longer affected by
limb-targeting penalty, because it was an accuracy debuff when there is
no inherent benefit to targeting Xeno limbs.
/:cl:

code/__DEFINES/mobs.dm

@@ -417,3 +417,4 @@ var/list/default_xeno_onmob_icons = list(
 #define HANDLING_LIMBS list("l_arm","l_hand", "r_arm", "r_hand")
 #define EXTREMITY_LIMBS list("l_leg","l_foot","r_leg","r_foot","l_arm","l_hand","r_arm","r_hand")
 #define CORE_LIMBS list("chest","head","groin")
+

code/modules/mob/mob_helpers.dm

@@ -28,7 +28,7 @@
 
 //TODO: Integrate defence zones and targeting body parts with the actual organ system, move these into organ definitions.
 
-//The base miss chance for the different defence zones
+/// The base miss chance for the different defence zones
 var/list/global/base_miss_chance = list(
 	"head" = 10,
 	"chest" = 0,

code/modules/projectiles/projectile.dm

@@ -455,12 +455,31 @@
 	if(hit_chance) // Calculated from combination of both ammo accuracy and gun accuracy
 
 		var/hit_roll = rand(1,100)
+		var/direct_hit = FALSE
 
-		if(original != L || hit_roll > hit_chance-base_miss_chance[def_zone]-20) // If hit roll is high or the firer wasn't aiming at this mob, we still hit but now we might hit the wrong body part
+		// Wasn't the clicked target
+		if(original != L)
 			def_zone = rand_zone()
+
+		// Xenos get a RNG limb miss chance regardless of being clicked target or not, see below
+		else if(isxeno(L) && hit_roll > hit_chance - 20)
+			def_zone = rand_zone()
+
+		// Other targets do the same roll with penalty - a near hit will hit but redirected to another limb
+		else if(!isxeno(L) && hit_roll > hit_chance - 20 - base_miss_chance[def_zone])
+			def_zone = rand_zone()
+
 		else
+			direct_hit = TRUE
 			SEND_SIGNAL(firer, COMSIG_BULLET_DIRECT_HIT, L)
-		hit_chance -= base_miss_chance[def_zone] // Reduce accuracy based on spot.
+
+		// At present, Xenos have no inherent effects or localized damage stemming from limb targeting
+		// Therefore we exempt the shooter from direct hit accuracy penalties as well,
+		// simply to avoid them from resetting target to chest every time they want to shoot a xeno
+
+		if(!direct_hit || !isxeno(L)) // For normal people or direct hits we apply the limb accuracy penalty
+			hit_chance -= base_miss_chance[def_zone]
+		// else for direct hits on xenos, we skip it, pretending it's a chest shot with zero penalty
 
 		#if DEBUG_HIT_CHANCE
 		to_world(SPAN_DEBUG("([L]) Hit chance: [hit_chance] | Roll: [hit_roll]"))