Remove the hardcoded (and apparently arbitrary) cap on oter_id

As mentioned by @acidia in pr #684, there was a hardcoded limit of 256-32 oter_id values. Implementation details: oter_id is an enum, and the main use case seems to be in a 2D oter_id array inside overmaps. As far as I'm aware, an array of enums would be 32 bits, so casting to a char doesn't actually save any bits.

In fact, as far as I can tell, the only reason we're capped at 256-32 oter_id values is because the save/load function casts them to chars. So, I've removed that artificial restriction.

It would probably still be good for someone who knows more about the overmap system to performance test this, just in case I've missed something.