RtAudio tlRender audio enhancements

[ggarra13]

Currently, RtAudio (on Windows and Linux) supports different back-ends. It would be great if it was possible to set the backend to use, to troubleshoot issues, like:

ggarra13/mrv2#194

On Windows, I would like to switch between Asio, WasApi and Direct Sound.
On Linux, we should be able to switch between JACK, ALSA and OSS.
I can probably add this myself, but you are better at designing the API than I am.

Also, on macOS, I had some requests for the following:

ggarra13/mrv2#221

I was not aware that RtAudio sets the audio to 48KHZ (on macOS at least) automatically. From what I can tell from looking at the source of RtAudio, there's currently no way to set the Audio Soundcard to.

mrViewer supported selection of audio soundcards, like:

[ggarra13]

Never mind. I went ahead and implemented it in mrv2.