Fix #10763, Fix #11168: Display variant groups that consist only of unavailable sub-groups

[merni-ns]

Motivation / Problem

Fixes #10763 and fixes #11168.

Essentially, the code added the immediate parent of a variant vehicle to the list, but not higher levels.

Description

Loop through and add all the parents of every variant vehicle up to the top level, so that even when a group consists only of unavailable sub-groups, the top-level group is still displayed when its second-level children are available.

The example provided by @George-VB in #11168 now produces the following list when game is started in 1.1.1970 with default settings.

And the example in #10763:

Limitations

Not well tested as I cannot write NewGRF nor do I know the actual intended output. Perhaps @George-VB could help with testing.

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

[merni-ns]

Ideally #11343 would be merged before this PR.
https://github.com/merni-ns/OpenTTD/blob/a80d8f6353d8c0a4bb4582afe8bf91d421d4c0aa/src/build_vehicle_gui.cpp#L1174
and
https://github.com/merni-ns/OpenTTD/blob/a80d8f6353d8c0a4bb4582afe8bf91d421d4c0aa/src/build_vehicle_gui.cpp#L1425
can be deleted once that is merged.

[PeterN]

This looks good to me, but needs #11343 to prevent a bad NewGRF creating an infinite loop.