audio: asio "downmix" 7.1 to stereo (#736)

## Link to GitHub Issue or related Pull Request, if one exists
#730 

## Description of change
Add an option that extracts channels from 7.1 ASIO and presents to 2-ch
ASIO. Same idea as the SDVX 2ch fix except for gitadora we are
duplicating the center channel to front.

## Testing
Tested SDVX and GFDM Arena.

src/spice2x/launcher/options.h

@@ -202,6 +202,7 @@ namespace launcher {
             AsioDriverName,
             AudioDummy,
             DownmixAudioToStereo,
+            AsioDownmixToStereo,
             VolumeBoost,
             AudioResample,
             AudioExclusiveBuffer,