avformat/matroskaenc: Put subtitles without duration into SimpleBlocks

avformat/matroskaenc: Put subtitles without duration into SimpleBlocks

The value zero for AVPacket.duration means that the duration is unknown,
which in practice means "play this subtitle until overridden by the next
subtitle". Yet for Matroska a BlockGroup with duration zero means
that the subtitle really has a duration zero. "Display until overridden"
is achieved by not setting a duration on the container level at all and
this is achieved by using a SimpleBlock or a BlockGroup without
duration. This commit implements this.

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