Feature: Close adjacent level crossings as if they were one large crossing

[Eddi-z]

Motivation / Problem

Level crossings on parallel tracks can be dangerous, especially for articulated road vehicles, or if multiple road vehicles follow each other. Often, a vehicle is waiting for a level crossing to open, when it suddenly gets struck in its "tail" by another train.

Description

Updated patch from https://www.tt-forums.net/viewtopic.php?f=33&t=46091

This patch tries to avoid the situation that a vehicle gets stuck inbetween two tracks by closing all level crossings simultaneously, and instead allowing road vehicles to leave a crossing they already entered. Best combined with a path signal some tiles ahead of a level crossing, to give vehicles time to leave if a train approaches.

Limitations

on loading an old savegame, road vehicles that were previously waiting for a crossing to open, may now immediately start up, causing them to run straight into a train. some savegame conversion update is needed.

also, a call to SndPlayTileFx(SND_0E_LEVEL_CROSSING, tile); fell off and i don't remember where it went, or why i removed it.
Update: level crossing sound is now changed to only appear when a train approaches the crossing, instead of sometimes when it is being closed.

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 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')
  • PR must be added to API tracker

[michicc]

Your missing SndPlayTileFx seems to be done in UpdateLevelCrossingTile. I haven't tried it yet, but I think it will now play a sound for each crossing tile that is closed, which should probably be reduced to only the triggering tile.

[Eddi-z]

I vaguely remember that there was a second source of that sound, when a train was approaching a crossing tile

[TrueBrain]

I kinda stopped my review half-way through. Please spend some more time on code quality here. So many things that appear to be in a half-state, that it made me check if this was a draft PR (it was not).

[Eddi-z]

it made me check if this was a draft PR (it was not).

it probably should be.

[Eddi-z]

i revisited the crossing sound effect. before this PR, crossing sound was played in two different ways

a) when a crossing without a PBS nearby gets closed
b) when a crossing with PBS is approached by a train

case a) was a bit awkward in my patch, since that might result in multiple crossing sounds being played at once, so i opted instead to drop case a) and change case b) to be in all cases

i opted to do this in a separate commit, in case it was somehow controversial