Add optional SDL3 support while keeping SDL2 as the default. SDL3 builds
must be selected explicitly with the SYSTEM3_SDL_VERSION CMake option.
Centralize most SDL2/SDL3 API differences behind compatibility interfaces.
Audio playback uses a dedicated SDL3 backend based on SDL3 audio streams,
implemented separately in mako_sdl3.cpp.
Android and Nintendo Switch remain on SDL2 for now.
- Invalid game id -> sys_error() in GameId::GameId()
- Unknown crc32 -> SDL_ShowSimpleMessageBox() in GameId::GameId()
- Cannot open ADISK.DAT -> sys_error() in NACT::NACT()
kichikuou