asfdec: subtract the preroll value and thus output 0 based timestamps

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/asfdec.c

@@ -236,6 +236,7 @@ static int asf_read_stream_properties(AVFormatContext *s, int64_t size)
     if (!asf_st)
         return AVERROR(ENOMEM);
     st->priv_data = asf_st;
+    st->start_time = 0;
     start_time = asf->hdr.preroll;
 
     asf_st->stream_language_index = 128; // invalid stream index means no language info
@@ -960,7 +961,7 @@ static int ff_asf_parse_packet(AVFormatContext *s, AVIOContext *pb, AVPacket *pk
             /* new packet */
             av_new_packet(&asf_st->pkt, asf->packet_obj_size);
             asf_st->seq = asf->packet_seq;
-            asf_st->pkt.dts = asf->packet_frag_timestamp;
+            asf_st->pkt.dts = asf->packet_frag_timestamp - asf->hdr.preroll;
             asf_st->pkt.stream_index = asf->stream_index;
             asf_st->pkt.pos =
             asf_st->packet_pos= asf->packet_pos;

libavformat/avformat.h

@@ -534,8 +534,6 @@ typedef struct AVStream {
      * Only set this if you are absolutely 100% sure that the value you set
      * it to really is the pts of the first frame.
      * This may be undefined (AV_NOPTS_VALUE).
-     * @note The ASF header does NOT contain a correct start_time the ASF
-     * demuxer must NOT set this.
      */
     int64_t start_time;