This isn't the greatest thing since it splits the A-to-B-Format transform from
the panning transform. The A-to-B and HF scale mixes are also not as optimal as
they could be, since they can't use the main mixer functions (wrong buffer line
length).
It does, however, get rid of the final use of the FOAOut buffer, so the
upsampling post-process is no longer needed.
5b35e60a98