Without the information, application may choose audio from one variant
and video from another variant, which leads to fetch two variants from
network. This enables av_find_best_stream() to find matching audio and
video streams, so that only one variant is fetched from network.
Signed-off-by: LYF <yefei.li@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>