Ability to capture audio from desktop capture on Web Audio backend

[vurpo]

Currently, miniaudio uses getUserMedia() to capture audio from audio input devices (usually microphones) in the browser. In the browser, it's also possible to use getDisplayMedia() to do desktop capture and get audio from audio output devices (kind of like a desktop loopback device).

The ability to choose between these two when running on the web platform would be very useful for my purposes.

[mackron]

Let me sit on this. I'm not massively enthusiastic about it, but at the same time it sounds like it would map relatively well to ma_device_type_loopback so I don't want to prematurely dismiss it. Will consider this. No time frame.