Fix #12497: [NewGRF] Increase vehicle motion counter for all parts.

[PeterN]

Motivation / Problem

#12229 stopped updating motion_counter for non-engine parts of trains, and in doing so accidentally followed the spec for NewGRF var 46, which breaks NewGRFs that used to accidentally work, but by the spec should not.

Fixes #12497.

Description

Some NewGRFs relied on the incorrectly motion counter that was 'fixed' in #12229, so always update v->motion_counter for all vehicle parts.

Always updating means that motion counters are still per-vehicle.

Limitations

Update motion counter for all vehicles might have a minor performance impact.

This still goes "against" the spec, but the spec could be updated to define this behaviour.

Alternatively we could leave #12497 option and let authors fix their sets.

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, game_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