fftools/ffmpeg: stop using decoder properties in ist_dts_update()

This code runs post-demuxing and is not synchronized with the decoder
output (which may be delayed with respect to its input by arbitrary and
unknowable amounts), so accessing any decoder properties is incorrect.

fftools/ffmpeg.c

@@ -1856,7 +1856,7 @@ static void ist_dts_update(InputStream *ist, AVPacket *pkt)
 
     if (!ist->saw_first_ts) {
         ist->first_dts =
-        ist->dts = ist->st->avg_frame_rate.num ? - ist->dec_ctx->has_b_frames * AV_TIME_BASE / av_q2d(ist->st->avg_frame_rate) : 0;
+        ist->dts = ist->st->avg_frame_rate.num ? - ist->par->video_delay * AV_TIME_BASE / av_q2d(ist->st->avg_frame_rate) : 0;
         if (pkt->pts != AV_NOPTS_VALUE) {
             ist->first_dts =
             ist->dts += av_rescale_q(pkt->pts, pkt->time_base, AV_TIME_BASE_Q);
@@ -1889,11 +1889,11 @@ static void ist_dts_update(InputStream *ist, AVPacket *pkt)
             ist->next_dts = av_rescale_q(next_dts + 1, av_inv_q(ist->framerate), time_base_q);
         } else if (pkt->duration) {
             ist->next_dts += av_rescale_q(pkt->duration, pkt->time_base, AV_TIME_BASE_Q);
-        } else if(ist->dec_ctx->framerate.num != 0) {
+        } else if (ist->par->framerate.num != 0) {
             int fields = (ist->codec_desc &&
                           (ist->codec_desc->props & AV_CODEC_PROP_FIELDS)) ?
                          ist->last_pkt_repeat_pict + 1 : 2;
-            AVRational field_rate = av_mul_q(ist->dec_ctx->framerate,
+            AVRational field_rate = av_mul_q(ist->par->framerate,
                                              (AVRational){ 2, 1 });
 
             ist->next_dts += av_rescale_q(fields, av_inv_q(field_rate), AV_TIME_BASE_Q);