mpeg2_metadata: Localize inserting of sequence display extensions

mpeg2_metadata: Localize inserting of sequence display extensions

If a new sequence display extension had to be added, this was up until
now done at two places: One where a sequence display extension was
initialized with default values and one where the actual sequence
display extension was inserted into the fragment. This division of
labour is unnecessary and pointless; it has been changed.

Furthermore, if a sequence display extension has to be added, the
earlier code set some fields to their default value twice. This has been
changed, too.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>