matroska: Add the CueDuration element

Use it only on subtitle CuePoints.
With proper demuxer/splitter support this should improve the display
of subtitles right after seeking to a given point in the stream.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
tools: Fix apparent merge failure