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