mxf: Add additional H264 PictureEssenceCoding

Unbreak some MXF Proxy files.
mxf: Override faulty PreviousPartition entries

Some files set the PreviousPartition field to point to its own offset.

If we are parsing forward the Previous partition is immediately known
and its value could be used, otherwise we can safely point to the
header.

Reported-By: Jean Baptiste Kempf <jb@videolan.org>
mxf: Verbose debug logging

Print the UL in code format and in the short format used by the
specification.
mxf: Lowercase UL strings

The specification uses lower case most of the time.