Fix: Support more than 256 stations/waypoints/roadstops per class.

[PeterN]

Motivation / Problem

It was already possible to define more than 256 per class, but not possible to use them as the index used in GUI and passed through commands was limited to a byte.

Objects are handled differently, by passing a global instead of class & class index, so do not have the same issue.

Loading lots of NewGRFs could cause more than 256 specs to be defined per class. This would then wrap around in the UI and now allow use of all specs.

468 waypoints are loaded, but only 256 are visible.

Description

Change the index to be uint16_t instead of a byte. While 65536 may be reachable it's not a problematic limit currently.

Same set of NewGRFs are loading which gives 468 waypoints:

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