if(pkt->dts < 0) pkt->dts= 0;
static void truncate_ts(AVStream *st, AVPacket *pkt){    int64_t pts_mask = (2LL << (st->pts_wrap_bits-1)) - 1;        if(pkt->dts < 0)        pkt->dts= 0;  //this happens for low_delay=0 and b frames, FIXME, needs further invstigation about what we should do here        pkt->pts &= pts_mask;    pkt->dts &= pts_mask;}