avformat/matroskaenc: Don't waste bytes on length fields

avformat/matroskaenc: Don't waste bytes on length fields

Several EBML Master elements for which a good upper bound of the final
length was available were nevertheless written without giving an
upper bound of the final length to start_ebml_master(), so that their
length fields were eight bytes long. This has been changed.

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