mxfdec: set audio timebase to 1/samplerate

Fixes sync in some samples (e.g. bugs 7581 and 8374 in VLC).
Based on a commit by Matthieu Bouron <matthieu.bouron@gmail.com>

Reported-by: Jean-Baptiste Kempf <jb@videolan.org>
CC: libav-stable@libav.org
Add support for multichannel ATRAC3+ streams.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
omadec: fix bitrate for ATRAC3+ streams

Signed-off-by: Anton Khirnov <anton@khirnov.net>