ffmpeg: stop using AVStream.codec on stream copy

This commit is based on commit 35c85806 from Anton Khirnov <anton@khirnov.net>
which was skipped in b8945c4e.

The avcodec_copy_context() call in the encode path is left in place for now
as AVStream.codec is apparently still required even after porting ffmpeg to
the new bsf API.

Tested-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>