MKV requires a unique(ish) TrackID element in the track info header.
Instead of the current hard-coded ID, take a hash of the video track
and use that. This value is not written in the deterministic output
mode, despite being a deterministic value itself, to give flexibility
to change the hash algorithm and not affect bisecting across the
change.

Change-Id: I807fc3ea6d1427a151c3ef703269b67e80aef860