avformat/asfdec: Use a parser for mpeg4-asp to find has_b_frames

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg: remove "has_b_frames" backpassing

It seems working without this now for the files i tested it with, if this causes
a regression, dont hesitate to put the line back or open a ticket or fix (if possible)
the parser

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg: request sample if the decoder & demuxer has_b_frames mismatch

We cannot use avpriv_request_sample() as this is private to the libs
or rather it would be a bad usage example

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>