Remove uniform submap branching

[prharvey]

Summary

Performance "Remove branching in submap accessors."

Purpose of change

See issue #70399

Describe the solution

Make uniform submaps point to a shared pre-filled submap, instead of having it be a special case. I'm not expecting any large performance changes from this alone, it's just easy to deliver in a PR separate from other tasks.

Describe alternatives you've considered

I did not change the get_items/get_fields non-const accessors since their behavior is somewhat odd, and has the potential to break things. They should be revisited (and removed) in a future PR.

Testing

All tests pass.

Additional context

[prharvey]

Going to leave this sitting until everything else in that issue is done, since I'm not yet completely convinced it's worth the added complexity.

[akrieger]

How much memory does this end up using? maptile_soa is not small, iirc.

[prharvey]

How much memory does this end up using? maptile_soa is not small, iirc.

IIRC, when I looked it was essentially the cost of 4 additional submaps (air, rock, soil, and one other). Given the reality bubble is ~2.5k submaps, it shouldn't matter.

[akrieger]

Uniform initialization is a nontrivial amount of time in memset as well during load. I will play with this and might ask to undraft this anyway.

Edit: eh, can't seem to pick it up anymore. Maybe other fixes when in which mitigated it (maybe it was happening too often before).

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