This will help elements that cannot deal with multistream,
such as the RTP payloader.
The caps now do not include a "streams" field anymore, but
a "multistream" boolean, since we have no real use for knowing
the exact amount of streams.
https://bugzilla.gnome.org/show_bug.cgi?id=665078
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
Additionally, since Opus packets may change between stereo
and mono as the encoder sees fit, assume two channels when
we do not see a header, and use the header value otherwise.
