Various places needed to differentiate better between "no duration is
known or set for this packet" and "this packet has a duration of
0ms". The former means that no duration element should be written to
the file, and the use of `SimpleBlock` instead of `BlockGroup` is
OK. The latter on the other hand means that a `BlockGroup` must be
used and a duration element must be added.
This core change uncovered a couple of subtle bugs in other places
where subtitles were handled:
• The Matroska reader passed 0 as the duration even if no duration
element was present making it impossible for other code to
differentiate between "no duration present" and "duration present
but set to 0".
• The DVB subtitle packetizer always enforced writing the duration
even if the duration wasn't known.
• The Kate packetizer used a wrong dummy value of 1us for the duration
for the "end of stream" packet as the core would not write a
duration of 0.
• The text subtitle packetizer was using the difference between the
current packet and the following one for packets were the duration
was 0 or unknown. The correct behavior is to do this only if the
duration is unknown, not if it is 0.
Fixes#2490.
