matroskamux: seeking messed up when remuxing H.264

Submitted by Tim Müller @tpm

Link to original bug (#639710)

Description

Remuxing H.264 from matroska to matroska like:

... demuxer.video_00 ! queue ! muxer.video_0 ...

creates a file with a broken index. When seeking, we don't seem to land on a keyframe, but any frame. After a few seconds, at the next keyframe, the garbage clears up again.