use reference counting on get_audio_device and get_midi_device

[andrewrk]

// TODO use reference counting instead so GenesisAudioDevice pointer never becomes invalid
struct GenesisAudioDevice *genesis_get_audio_device(struct GenesisContext *context, int index);

// TODO use reference counting instead so GenesisAudioDevice pointer never becomes invalid
struct GenesisMidiDevice *genesis_get_midi_device(struct GenesisContext *context, int index);