avformat/oggdec: dont read timestamps from EOS pages of ogm videos

Some muxers store invalid timestamps there, which breaks seeking
Fixes Ticket2739

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/libopusdec: Set codec->delay to pre_skip not fixed value

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
HACK mov: don't set the DEFAULT disposition because of wrong semantics

The meaning of the DEFAULT flag and movs enabled flag don't match and
cause playback issues.