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
..
asf
2012-09-28 02:40:53 +02:00
au
bmp
dpx
2012-08-27 18:31:21 +02:00
mkv
2012-10-15 09:19:21 +02:00
mov
2012-10-12 20:16:48 +02:00
ogg
2012-07-04 16:20:47 +02:00
pam
2012-05-15 19:47:18 +01:00
pcx
2012-10-13 09:07:12 +00:00
pgm
png
2012-10-13 16:02:09 +00:00
ppm
sgi
2012-03-03 20:57:03 -05:00
tga
xbm
2012-04-09 17:21:35 +02:00