Extend Audio interface to support Xbox One headset bringup

[endrift]

I'm working on adding support for various Xbox One controller attachments, with headset support being the obvious next one. However, per @slouken, the SDL audio API doesn't have all the functionality needed for this:

I'd open an issue and ping @icculus. The basic idea is we need to extend the audio driver code to support multiple concurrent drivers, or split it so we can add external audio devices with appropriate callbacks.

Once the interface additions are merged I can start work on adding headset support.

[slouken]

@icculus, this is an interesting one. It's an audio device created and removed at runtime from external code. Maybe we want the concept of a virtual audio device that we could create dynamically?

[icculus]

It's entirely userspace? No WASAPI or DirectSound interface to it? Then yeah, a virtual device thing is going to be the way to go.

[slouken]

It's entirely userspace? No WASAPI or DirectSound interface to it? Then yeah, a virtual device thing is going to be the way to go.

Yep!

[endrift]

To be extra clear here: while Windows does provide an interface to it, this driver is meant to run in the stead of the Windows driver, at least in some contexts.

[endrift]

Looking over this a bit more, it looks like the protocol to the controller requires isochronous transfers. Is that possible to do with SDL's libusb interface?