It's treated as 5.1 + 2 aux channels. This allows AL_DIRECT_CHANNELS_SOFT to
behave better, not forwarding rear left/right channel inputs to lower front and
upper rear, and allows reporting a more appropriate output mode to the app
instead of 7.1.
It messes with 5.1 sources using direct channels, and the surround channels are
supposed to map to the side labels. Individual backends can deal with the
channel order/label differences, as they already do to a degree.
