demux: matroska: mkv files generated with GStreamer and streamable=true are not playable in VLC

A file generated with: gst-launch-1.0 -e videotestsrc ! vp8enc ! matroskamux streamable=true ! filesink location=testsrc.mkv

fails to play with:

[00007f0780c02240] mkv demux error: unable to read KaxCluster during seek, giving up
[00007f0780c02240] mkv demux error: unable to read KaxCluster during seek, giving up

I'm not sure if this is a Gstreamer problem, a VLC problem, or both.

Note that it is playable with: vlc --demux avformat testsrc.mkv

or if I generate the file without streamable=true.

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