Add support for record system audio on macOS

[thewh1teagle]

Hey!
I would like to use rtaudio in my desktop app for record mic / speakers output on macOS / Windows.
I found that it already supports recording speakers on windows using WSAPI.
In recent versions of macOS, there's new API called screen capture kit which provides a way getting audio samples from the speakers.
I'm not sure if Core Audio supports it, but screen capture kit definitely support it.
That way other apps can use rtaudio to record system audio and potentially their mic in the same stream, and it's useful for record meetings / screen recording and there's many more use cases.
Thanks

[SuibianP]

+1, would be nice to have a cross-platform way to capture system audio as well. It should also be possible for PulseAudio with pa_stream_set_read_callback.