Support a separate mixer_device configuration file option for
advanced ALSA configurations. Previously, ALSA local output
happened to work becasue "default" is valid as both a PCM and a
mixer. Now you can separately specify the device name for PCM
output and mixer operations.
In my setup, I am using the following setup:
card = "default:CARD=NVidia"
mixer = "Front"
mixer_device = "hw:CARD=NVidia"