opus: properly create channel mapping tables

opus: properly create channel mapping tables

There are two of them, unintuitively enough; the one passed
to the encoder should not be the one that gets written to the
file. The former maps the input to an ordering which puts
paired channels first, while the latter moves the channels
to Vorbis order. So add code to calculate both, and we now
have properly paired channels where appropriate.

https://bugzilla.gnome.org/show_bug.cgi?id=665078