Up until now, the Matroska muxer writes only one Tags level 1 element
and therefore using a certain place to store the dynamic buffer used for
writing it was hardcoded; yet the Matroska specifications allow an
unlimited amount of Tags elements and we have reason to write a second
one: If chapters are provided after writing the header, they are written
when writing the trailer; yet the corresponding tags are ignored. This
can be fixed by writing them in a second Tags element.

Also use a MatroskaMuxContext * instead of an AVFormatContext * as
parameter in mkv_write_tag() and mkv_write_tag_targets() as that is all
these functions use.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>