Sometimes, pools would spawn deep underground and then produce deep holes.
I noticed that such pools would be at +48, at the block boundary.
IMHO this may be an error in minetest surface detection.
Nevertheless, here is a workaround: require air above pool spawns.
Also clean up the pool code
Reviewed-on: VoxeLibre/VoxeLibre#4571
Reviewed-by: Mikita Wiśniewski <rudzik8@protonmail.com>
Co-authored-by: kno10 <erich.schubert@gmail.com>
Co-committed-by: kno10 <erich.schubert@gmail.com>