Capture playback devices on wasapi

[playmer]

On the WASAPI backend, you can now call SDL_OpenAudioDevice for capture on playback devices.

Description

• Modified SDL_AudioDriverImpl to allow backends to declare the capability to capture playback devices.
• Modified WASAPI backend to declare the above capability.
• Modified SDL_audio to allow users to record Playback devices when the backend declares this capability
• Modified WASAPI backend to pass the LOOPBACK flag when capture is requested on a Playback device.

Existing Issue(s)

#7691

[slouken]

Hi! :)

[icculus]

Hi! :)

Was that for me? :)

So what I want to do is make this use the same hint PulseAudio does: SDL_HINT_AUDIO_INCLUDE_MONITORS, to say "I want the output of other audio devices as a capture device too," which is exactly what we're doing here.

But also, the whole audio subsystem is about to look completely different when #7704 merges, so we should hold off on this a little longer and then incorporate the meat of this patch in once that's done.

If this is urgent, then let's just merge it now and I'll do the reworking mentioned above as part of #7704.

[playmer]

The hi was for verification on the SDL discord, we were slightly concerned about impersonation. Sorry for inadvertently getting your attentions icculus! This isn't urgent!

[icculus]

The new audio subsystem is merged, so I expect to play around with this PR pretty soon. Just giving a status update here, you haven't been forgotten. :)

[icculus]

Still want this, but I'm going to kick this to 3.2.0 (since it's not an ABI change)...but definitely want some form of this in the official first release.