avformat/matroskaenc: Improve BlockAdditions

8ffcc826 added support for muxing BlockAdditions with BlockAddID equal
to one. The restriction to BlockAddID == 1 probably resulted from
a limitation to what was needed; yet over time this led to three
occurences of "(side_data_size && additional_id == 1)". This commit
changes this by setting side_data_size to 0 if additional_id != 1.

It also stops hardcoding 1 for the value of BlockAddID to write;
but it still upholds the requirement that it is 1. See below.

Despite BlockAddId actually having a default value of 1, it is still
written, because until very recently (namely dbc50f8a) our demuxer
used a wrong default value of 0.

Furthermore, use put_ebml_binary() to write the BlockAdditional element.

(The Matroska specifications have evolved and now the BlockAddID 1 is
reserved for the codec (as described in the codec's codec mapping),
BlockMore elements with BlockAddID > 1 are now of a more
codec-independent nature and require a BlockAdditionalMapping in the
track's TrackEntry. Given that this muxer does not support writing said
BlockAdditionalMapping yet (actually, none have been defined yet), we
have to uphold the requirement that BlockAddID == 1.)

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

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