This should cause maps that have the same
seeds to have much more similar results
than before, e.g. sponge deposits will be in
the same places determined by seed and not
random each regeneration of the map.
- Mapgen shared now provides an RNG which
will be deterministic when feasible, for
repeatable mapgen results.
- Make existing rng-using mapgen hooks use
the new deterministic RNG.
- Mapgen shared hooks are also run in
deterministic order too.
- Tidy up mapgen_shared API a little more.
