Codechange: use std::variant instead of using bitflags in the value

[TrueBrain]

Motivation / Problem

While working on #11190 I found that InitializeViewport used some weird tactics to support a Vehicle viewport and a Tile viewport. Let's modernize it a bit!

Description

Use std::variant to know whether we want to see a Vehicle or a Tile. This works, as TileIndex is using StrongTypeDef and as such, is another type from VehicleID (which is just an integer).

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, 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

[PeterN]

oooh shiny modern!