Despite the suggestive size limits, this metadata ID has nothing to do
with the VDR metadata ID used for the data mappings. Actually, the
specification leaves them wholly unexplained, other than acknowleding
their existence. Must be some secret dolby sauce. They're not even
involved in DM metadata compression, which is handled using an entirely
separate ID.

That leaves us with a lack of anything sensible to do with these IDs.
Since we unfortunately only expose one `dm_metadata_id` field to the
user, just ensure that they match; which appears to always be the case
in practice. If somebody ever hits this error, I would really much
rather like to see the triggering file.