Support downmixing audio to mono

[Splendide-Imaginarius]

For accessibility reasons, it would be helpful to support optionally downmixing stereo BGM to mono. I suspect OpenAL should make this straightforward.

(h/t @JaidenAlemni)

[JaidenAlemni]

This would indeed be a very helpful feature. I would just see it as a JSON configuration option as a boolean, maybe "forceMono". That would make the Ruby-side implementation a trivial configuration hash update. I'm not sure it really needs to be expanded beyond that (I see no reason to separate SE from BGM, for example)?

[Splendide-Imaginarius]

I can picture some use cases (including one of mine) where BGM and SE might benefit from different behavior, but besides that, yes agreed that a boolean JSON config option should do the job.