C++20

[vkedwardli]

• Enabled C++20 for Flycast project, dependencies are still C++17
• Added fmtlib to replace std::format (which is not supported on macOS < 13.3)
• UWP: C++/WinRT migration
• Added /WHOLEARCHIVE for SDL2 to fix metadata stripping in C++20 builds
• Updated DreamPicoPort-API to fix C++20 template errors (will wait for upstream)

Following PR #1696, I thought I could use designated initializers since the project is now C++20, but MSVC is very strict with the feature set. This is another attempt to fix what PR #1868 missed.

Looking for feedback on these C++20 workarounds and would like to confirm if this strategy aligns with the project's direction.

I concern that with C++20 enabled, contributors will naturally use std::format, they might wait hours for a full build only discover the macOS CI to fail late in the process, so

• Proposing fmtlib as the project-wide formatting standard
  • Uniform behavior across MSVC/GCC/Clang, supports older macOS (< 13.3) where std::to_chars is missing
  • better compile-time format checks (FMT_STRING)
  • better performance tuning options
• Should we block std::format via Header Guards so whenever someone try to compile, so error pops up immediately? Or pre-commit hook with CMake script?
• UWP / SDL2 / WHOLEARCHIVE: Since we are still using SDL2, unless we want to upgrade to SDL3

p.s. The UWP build can be run on a Windows 11 machine, can play games, not sure what actions are required to verify the UWP build since I don't have an Xbox

[vkedwardli]

• Rebased to include the commit 6d2befc
• Fixed the HiDPI + mouse hit-testing issue observed on Win11 UWP builds also

Smoke Test (Win11 UWP)

• App launches successfully (no crash)
• File picker works (load game)
• Data persists across restart
• Screenshots save correctly (multiple captures)
• HTTP requests work (album art loads)
• Keyboard input works (InputPane)
• Game runs normally
• HiDPI scaling correct on first launch
• Mouse input works (hover + click alignment correct)

Before
Win11:

• Always used 640×480 or saved size directly
• DPI scale always 1.0
• Result: blurry UI + broken mouse mapping (offset)

Xbox:

• HDMI success -> OK
• HDMI failure -> fallback to 640×480 (no recovery)

After
Win11:

• Always uses OS DPI scale
• Correct physical resolution every launch

Xbox:

• HDMI success -> unchanged
• HDMI failure -> fallback to OS scale instead of 640×480