Clément Bœsch b08273c9ca lavf/mkv: avoid negative ts by default.
This fixes playback in some circumstances (like webm in firefox).
Regression after 2c34367b.

It is also matching the Matroska specifications:
http://matroska.org/technical/specs/notes.html, "The quick eye will
notice that if a Cluster's Timecode is set to zero, it is possible to
have Blocks with a negative Raw Timecode. Blocks with a negative Raw
Timecode are not valid."
2012-10-15 09:19:21 +02:00
..
2012-09-28 02:40:53 +02:00
2012-05-17 02:34:57 +01:00