avformat/rpl: use avpriv_report_missing_feature/avpriv_request_sample

Signed-off-by: Paul B Mahol <onemda@gmail.com>
matroskadec: Demux support for SeekPreRoll and CodecDelay

Adds support for demuxing SeekPreRoll and CodecDelay container
elements.

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>