Motivation / Problem

The format codes for printf are complicated and annoying to work with. When changing something from C-string to std::string you need to explicitly add .c_str(), but also for some types you need to "inject" PRINTF_SIZE.

Preferably we would use something in the standard library, but formatting has landed in C++20 so... almost there but not quite yet. There is however the "fmt" library that has the same API as C++20 just in a different namespace, so we can use that as a stop-gap measure until we go to C++20. See https://fmt.dev/7.1.3/api.html#compatibility-with-c-20-std-format

Description

The vendoring of the "fmt" library and full conversion of all DEBUG statements to Debug.

Limitations

• ~9000 lines of extra code due to the vendoring of the library.
• There is a set of fmt::printf functions, however they do not support things like %ld so using that as a migration step is out of the question.
• The error messages on wrong formats can be very unhelpful, though it does have compile time validation of the format strings.
• There are a lot of inconsistencies in the debug messages. Those are not in scope here, unless someone wants to patch that on top of this. That I'll gladly accept.

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 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