mp3: enable packed main_data decoding in MP4

14496-3 suggests packing main_data of MP3 that is usually scattered
into multiple frames due to bit reservoir.

However, after packing main_data into a access unit, bitrate index
in the MPEG audio frame header doesn't match with actual frame size.

In order to accept this, this patch removes unnecessary frame size
checking on mp3 decoder.

Also, mov demuxer was changed to use MP3 parser only on special cases
(QT MOV with specific sample description) to avoid re-packetizing.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>