[opus] Force Opus/CELT to encode 2 mono instead of 1 stereo

[Andrew P. Lentvorski]

Hi, folks,

I have an old CELT thing that I'm updating to Opus and I'm trying to get
individual pieces upgraded.  My first task is upgrading the CELT encoder
to Opus.

The old CELT thing treated the L and R channels as independent mono
streams, encoded them somehow with CELT to form one bitstream, sent one
bitstream across, and then unpacked it manually after the decoder.

Can I do that with Opus?

Right now, I'm choosing OPUS_APPLICATION_RESTRICTED_LOWDELAY in order to
only use the CELT subsystem.  However, I'm getting a lot of bleedthrough
on the stereo channels which I didn't get before.  Is there a magic
setting to adjust this?

Thanks,
-a