[AD] audio panning

[Peter Wang <novalazy@xxxxxxxxxx>]

The kcm_audio interface currently does not support panning samples.
With multi-channel audio it's not obvious how to add it, or even what it
should do.  Do we place a sample on two axes, left-right and front-back,
or even three?  Or do we simply let the user specify the relative
scaling factors for each output channel?

I'll look around at other libraries, too.

Peter