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.
Before this, MIDI playback was done by generating an SMF data stream,
which was then played using platform-specific methods (MCI on Windows,
MediaPlayer on Android, SDL_Mixer on other platforms). However, this
approach had an issue where the Y18 command (which reads loop counts and
markers) did not work correctly.
With this change, the MAKOMidi class now uses the RtMidi library to
control MIDI devices in real-time. This approach was used in the
original "System3 for win32", but has now been made cross-platform.
The dependency on RtMidi is optional. If RtMidi is not available at
build time or a MIDI device is unavailable at runtime, FM synthesis will
be used.
kichikuou