avformat/mxfdec: use both body_sid and track_number to find the track of a packet

      In order to do that we have to parse the EssenceContainerData and assign the
proper body_sid and index_sid to the tracks from the corresponding source
packages.

This fixes packets returned in the wrong stream for some OP1-b files.

Based on a patch by Alex Mogurenko from https://github.com/da8eat/FFmpeg

Reference: http://mogurenko.com/2018/01/02/mxf-op1b-ffmpeg-part1/

Signed-off-by: Marton Balint <cus@passwd.hu>