af_hdcd: Process stereo channels together, fix #5727

Issue #5727: gain adjustment should only be applied if matching
gain value from a valid packet in both channels. The existing functions process
each channel separately, so it was not possible.

* New versions of hdcd_process(), hdcd_scan(), hdcd_integrate() named
  hdcd_*_stereo() that process both channels together.
* target_gain applied will be the last matching target_gain.
* The old single channel functions remain as an option. They can be
  used by: -af hdcd=process_stereo=0.

Signed-off-by: Burt P <pburt0@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
