Add option to be able to duplicate stereo sources to back channels

[chros73]

The music mode of my AVR converts 2ch sources to 7.1 and I hate the center channel sounds. It can be also useful for analogue output with 4/6 speakers when the sound driver doesn't support mirroring.

Currently if I select Quadrophone in the speaker setup (Windows) only the front 2 sounds -> that means windows won't upmix stereo signal to the selected speaker config.
Option 1 would be asking wasapi to do so (using the system default speaker config).
But Option 2 would be more flexible: the plugin could determine the max speaker config and automatically select the right config or allow to set smaller than or equal amount (if it's achievable).
I think it doesn't concern downmixing.
This option can also exist with Exclusive mode as well not just shared one.

That's the only thing that I'm missing.
Thanks