Add subways and layers

[GMart]

I see there is already a PR adding subways. I think I can merge that one in, as the look is a little better than mine. I have also added subway stations and train stops. Big change with this one, in the way that data processing works. In order for underground structures to generate with air blocks, I put the fill ground portion before the element processing. That way, the element processing can make air blocks as needed. This means the underground structures work when fill ground or terrain is disabled or enabled, it doesn't matter. I also respect the level tags, so subways that cross under each other show correctly in Arnis. However, they abruptly go from one level to the next, which can be improved.

Todo:

• [x] Make stations, stops, etc. generate as areas instead of ways.
• [ ] Make levels flow into each other, instead of jumping from one level to the next.
• [x] Add station entrances. Will probably not work correctly everywhere since they are defined in OSM differently.

There are a few bugs with formatting to work out.

[GMart]

Screenshot here: https://github.com/louis-e/arnis/issues/418#issuecomment-2844832052

[GMart]

retrigger-benchmark

[github-actions[bot]]

⏱️ Benchmark run finished in 0m 39s 🧠 Peak memory usage: 1611 MB

📈 Compared against baseline: 42s 🧮 Delta: -3s 🔢 Commit: 71e28d1

🟢 Generation time is unchanged.

You can retrigger the benchmark by commenting retrigger-benchmark.

[GMart]

Current bugs with this fork:

• The progress bar is messed up since I switched the order of generation, ground generation happens first.

• Underground roads can intersect with subways but the subway walls are always generated in the way. It may not be possible to determine when the path should be open to the subway. Usually, stations generate correctly, next to the subway line.

• Subways can look weird with both terrain and fill ground on.

• Subway entrance openings don't always generate on the surface.

[GMart]

When I have time, I will separate this out into two PRs, one for the Subways and underground, and one for modifying how the generation works so that underground structures generate with fill ground on.

[louis-e]

Hi GMart, thanks a lot for your work! FYI I'm currently working on finishing up everything for the next major release. Originally I was planning on including the subway feature for this release, but I also struggled with integrating it, so I appreciate you spending time on that. Depending on when the release will happen, we can include this either in the major release or as a minor release. :)