Using lowest PTS or DTS is a bad idea, as lower video packets are sent
ahead of pts, and most audio tracks are sending packets near to pts.
So we had audio packets coming late, with a lower pts, and corrupted
our PCR as we take the lowest pts among tracks.
We can't also guess the full tracks rolls, so updating after a full
roll instead of each packet is useless.
Send time is a reliable base time.