Simple animals don't block projectiles after death anymore (#6522)

# About the pull request
Simple animals don't block projectiles after death anymore 

# Explain why it's good for the game
SADARing a rat corpse is fun but only a few times.


# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog
:cl: ihatethisengine2
fix: simple animals don't block projectiles after death anymore 
/:cl:

code/__DEFINES/mobs.dm

@@ -380,5 +380,6 @@ GLOBAL_LIST_INIT(default_xeno_onmob_icons, list(
 #define MOBILITY_LIEDOWN (1<<8)
 
 #define MOBILITY_FLAGS_DEFAULT (MOBILITY_MOVE | MOBILITY_STAND)
+#define MOBILITY_FLAGS_LYING_CAPABLE_DEFAULT (MOBILITY_MOVE | MOBILITY_STAND | MOBILITY_LIEDOWN)
 #define MOBILITY_FLAGS_CARBON_DEFAULT (MOBILITY_MOVE | MOBILITY_STAND | MOBILITY_REST | MOBILITY_LIEDOWN)
 #define MOBILITY_FLAGS_REST_CAPABLE_DEFAULT (MOBILITY_MOVE | MOBILITY_STAND | MOBILITY_REST | MOBILITY_LIEDOWN)

code/modules/mob/living/simple_animal/friendly/mouse.dm

@@ -77,6 +77,7 @@
 	src.icon_dead = "mouse_[body_color]_splat"
 	src.icon_state = "mouse_[body_color]_splat"
 	layer = ABOVE_LYING_MOB_LAYER
+	set_body_position(LYING_DOWN)
 	if(client)
 		client.time_died_as_mouse = world.time
 

code/modules/mob/living/simple_animal/simple_animal.dm

@@ -56,6 +56,8 @@
 	black_market_value = 25
 	dead_black_market_value = 0
 
+	mobility_flags = MOBILITY_FLAGS_LYING_CAPABLE_DEFAULT
+
 /mob/living/simple_animal/Initialize()
 	. = ..()
 	SSmob.living_misc_mobs += src
@@ -210,6 +212,7 @@
 	SSmob.living_misc_mobs -= src
 	icon_state = icon_dead
 	black_market_value = dead_black_market_value
+	set_body_position(LYING_DOWN)
 
 
 /mob/living/simple_animal/gib(datum/cause_data/cause = create_cause_data("gibbing", src))