matroskademux: Advance sparse streams only as much as required to keep the gap smaller than 500ms
Changing it to the newest timestamp that was ever pushed will increase the segment start in 500ms jumps, which could be just after the next sparse stream buffer. E.g. Video at 1.0s, sparse stream at 0.5s would jump the sparse stream to 1.0s. Now a new sparse stream buffer could appear that has a timestamp of 0.9s and this would be dropped for no good reason because of bad luck.