Add: [YAPF] Rail depot penalty based on the number of trains waiting in it

[nchappe]

Motivation / Problem

Non-stopped trains waiting in depot are not taken into account in YAPF cost calculations, because they may not have any track reserved. This can for instance be a problem for 'go to nearest depot' orders, as a train may choose to go to a depot full of waiting trains when it could go to a slightly more distant empty depot instead.

Description

This PR adds a penalty for trains in depot to YAPF.
rail_pbs_cross_penalty seemed to be the most relevant penalty as it is used for reserved tracks.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[2TallTyler]

I'm in favor of this, but I recognize that it could be confusing to players when a vehicle bypasses a nearby depot to go further away. I'd be curious to see what happens when a large number of vehicles is in a depot (say, after cloning 20+ trains and starting them all) — maybe a maximum penalty would be a good idea, if there's any precedent for that in other penalty calculations.

[nchappe]

To answer your question, the default value of rail_pbs_cross_penalty is 3 * YAPF_TILE_LENGTH so after cloning 20 trains, a train going to the nearest depot would prefer an empty depot 60 (free) tiles away.

I agree this can be confusing, so I added a cap of 8 trains. Beyond 8 trains stuck in the same depot, I guess the player should probably rethink their network anyways.
The max "trains in depot" penalty is now equivalent to a distance of 24 free tiles, or (I think) 6 occupied tiles.

[PeterN]

Building lists of vehicles in a depot within the pathfinder seems like it might be bad idea for pathfinder performance.

[2TallTyler]

Maybe having depots cache the current number of non-stopped trains inside them would be a better solution.

[2TallTyler]

@nchappe Any plans to resume work on this based on the review comments?