Fix #10395: When loading old saves, don't forcibly bar level crossings

[2TallTyler]

Motivation / Problem

The savegame conversion in #9931 iterates through all level crossings and updates them, in case the savegame had only one track of a multi-track crossing barred. I erroneously forced every level crossing tile on the map to be barred instead of checking for a closed tile on the same axis. Reported as #10395.

Description

Check the crossing for barred tiles instead of forcing all tiles to be barred.

Closes #10395.

To make review a bit easier, the parameter being changed is force_bar:

/**
 * Update a level crossing to barred or open (crossing may include multiple adjacent tiles).
 * @param tile Tile which causes the update.
 * @param sound Should we play sound?
 * @param force_bar Should we force the crossing to be barred?
 */
void UpdateLevelCrossing(TileIndex tile, bool sound, bool force_bar)

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