mxfdec: Speed up metadata and index parsing.

Specifically, this means parsing as before until we run into essence.
At that point we seek to the footer and parse until EOF. After that we start
seeking backward to the previous partition and parse that until we run into
essence or the next partition. This procedure is repeated until we encounter
the last partition we parsed in the forward direction.

The end result of all this is that large essence containers are not needlessly
parsed. This speeds up parsing large files a lot.

Signed-off-by: Janne Grunau <janne-libav@jannau.net>

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