smoothstreamingenc: Don't assume streams start from timestamp 0

Also use dts instead of pts for deciding where to split fragments.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/smoothstreamingenc.c

@@ -559,12 +559,15 @@ static int ism_write_packet(AVFormatContext *s, AVPacket *pkt)
     SmoothStreamingContext *c = s->priv_data;
     AVStream *st = s->streams[pkt->stream_index];
     OutputStream *os = &c->streams[pkt->stream_index];
-    int64_t end_pts = (c->nb_fragments + 1) * c->min_frag_duration;
+    int64_t end_dts = (c->nb_fragments + 1) * c->min_frag_duration;
     int ret;
 
+    if (st->first_dts == AV_NOPTS_VALUE)
+        st->first_dts = pkt->dts;
+
     if ((!c->has_video || st->codec->codec_type == AVMEDIA_TYPE_VIDEO) &&
-        av_compare_ts(pkt->pts, st->time_base,
-                      end_pts, AV_TIME_BASE_Q) >= 0 &&
+        av_compare_ts(pkt->dts - st->first_dts, st->time_base,
+                      end_dts, AV_TIME_BASE_Q) >= 0 &&
         pkt->flags & AV_PKT_FLAG_KEY && os->packets_written) {
 
         if ((ret = ism_flush(s, 0)) < 0)