avcodec/dovi_rpudec: implement limited DM decompression

This implements the limited DM metadata compression scheme described in
chapter 9 of the dolby vision bitstream specification.

The spec is a bit unclear about how to handle the presence of static
metadata inside compressed frames; in that it doesn't explicitly forbid
an encoder from repeating redundant metadata. In theory, we would need
to detect this case and then strip the corresponding duplicate metadata
from the existing set of static metadata. However, this is difficult to
implement - esspecially for the case of metadata blocks which may be
internally repeated (e.g. level 10).

That said, the spec states outright that static metadata should be
constant throughout the entire sequence, so a sane bitstream should not
have any static metadata values changing from one frame to the next (at
least up to a keyframe boundary), and therefore they should never be
present in compressed frames. As a consequence, it makes sense to treat
this as an error state regardless. (Ignoring them by default, or
erroring if either AV_EF_EXPLODE or AV_EF_AGGRESSIVE are set)

I was not able to find such samples in the wild (outside of artificially
produced test cases for this exact scenario), so I don't think we need
to worry about it until somebody produces one.

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