Animates the disappearance of the lower half of the body when north-side leaning (#22568)

fixes #22249 

- bugfix: "Leaning on the north side of a wall now correctly hides your
lower body behind the wall instead of drawing you on top of it."
- code_imp: "Adds a COMSIG_MOB_LYING_DOWN signal, sent when a mob
transitions into lying down."

Cutout was previously only cutting out the base because KEEP_TOGETHER
wasn't being applied. Adds the flag on lean, removes it on stop_lean.

Animated the cutout while here. Applies a mask to the whole sprite, then
animates it upwards in step with the lean itself, so the feet
progressively disappear as they should.


https://github.com/user-attachments/assets/fec0a852-247a-4570-a6d1-60dda6fe6a3c

code/modules/mob/mob.dm

@@ -916,6 +916,8 @@
 
 	if( lying != lying_prev )
 		update_icon()
+		if(lying)
+			SEND_SIGNAL(src, COMSIG_MOB_LYING_DOWN)
 
 	return canmove