Channel mapping 2 additionally supports a non-diegetic stereo track
appended to the end of a full-order ambisonics signal, such that the
total channel count is either
  (n + 1) ^ 2, or
  (n + 1) ^ 2 + 2
where n is the ambisonics order
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>