s->mb_skiped = 0;
void MPV_frame_start(MpegEncContext *s){    int i;    UINT8 *tmp;    if (s->pict_type == B_TYPE) {        for(i=0;i<3;i++) {            s->current_picture[i] = s->aux_picture[i];        }    } else {        for(i=0;i<3;i++) {            /* swap next and last */            tmp = s->last_picture[i];            s->last_picture[i] = s->next_picture[i];            s->next_picture[i] = tmp;            s->current_picture[i] = tmp;        }    }}