When a slime or magma cube dies, it should spawn between 2 to 4 smaller
children. The code was always spawning 4 children (3 for big magma
cubes). This commit makes the following changes to the function
`spawn_children_on_die`:
* Make the number of children spawned a random number between 2 and 4.
* No longer accept the `children_count` as an argument, because this
number should always be a random number between 2 and 4 anyway.
* Update all callers accordingly.
95cbac78a8