ffmpeg: use avformat_seek_file() instead of av_seek_frame().

avformat_seek_file() is the new API. It will make sure the read_seek2()
callback is called when the demuxer has it.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Clément Bœsch <ubitux@gmail.com>