Implement alternative MIDI backends

[schlangz]

Current implementation uses WildMIDI, would be nice to have support for system MIDI devices.
https://github.com/thestk/rtmidi looks good from what I have seen so far.

Let me add more details: from what I understood is that WildMIDI uses GUS patches, but I'm very certain that over 99% of the people never played this game with GUS MIDI music and probably sounds foreign to them (me included). That is why adding support for selectable OS MIDI devices would be nice to bring back the music the people are used to.

[schlangz]

#236 submitted

Adds two new music source options to the debug overlay dropdown (MIDI / MIDI OS / MIDI OPL3 / CD):

MIDI (OS) -- sends MIDI data to the OS hardware/software synthesizer via RtMidi (WinMM on Windows). Vendored in external/rtmidi/.

MIDI (OPL3) -- synthesises HMP files to PCM using libADLMIDI 1.6.2 with a DosBox OPL3 emulator, producing authentic DOS Sound Blaster sound with no OS MIDI infrastructure required. Works on Android. Vendored in external/libadlmidi/ (DosBox emulator only; Nuked/Opal/Java disabled).

Both backends share the same start/stop/volume/config wiring as the existing WildMIDI backend.
Music source persists across sessions via the MIDI_OS / MIDI_OPL config tokens.

Edit: Android lesson learned

[schlangz]

I was beautifying the debug overlay combos and ran into random crashes when selecting combo options that pointed to Nuklear implementation.
Submitted Nuklear PR Immediate-Mode-UI/Nuklear#968 and ROLLER PR #238