avformat/mux: 2 subtitle packets could have the same DTS

Fixes Ticket3514

See: ETSI EN 300 743 V1.3.1 (2006-11)
"In summary, all of the segments of a single display set shall be carried in one (or more) PES packets that have the same
 PTS value."
with PTS = DTS and remuxing of such a stream it is to be expected that sometimes
multiple packets would have the same DTS

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>