avformat/hlsenc: compute segment duration use current pts minus last segment end pts

segment duration is using vs duration which compute by frame per second,
that can not fix problem of VFR video stream, so compute the duration
when split the segment, set the segment target duration use
current packet pts minus the prev segment end pts..
Reported-by: Zhao Jun <barryjzhao@tencent.com>
Reviewed-by: Zhao Jun <barryjzhao@tencent.com>
Signed-off-by: Steven Liu <liuqi05@kuaishou.com>

libavformat/hlsenc.c

@@ -2463,7 +2463,8 @@ static int hls_write_packet(AVFormatContext *s, AVPacket *pkt)
         }
 
         if (vs->start_pos || hls->segment_type != SEGMENT_TYPE_FMP4) {
-            ret = hls_append_segment(s, hls, vs, vs->duration, vs->start_pos, vs->size);
+            double cur_duration =  (double)(pkt->pts - vs->end_pts) * st->time_base.num / st->time_base.den;
+            ret = hls_append_segment(s, hls, vs, cur_duration, vs->start_pos, vs->size);
             vs->end_pts = pkt->pts;
             vs->duration = 0;
             if (ret < 0) {