ffmpeg: improve the intra stream discontinuity message

Now it actually tells which stream from which input and of
which type had an absolute DTS discontinuity larger than
dts_delta_threshold.

fftools/ffmpeg.c

@@ -4471,7 +4471,10 @@ static int process_input(int file_index)
                 pkt_dts + AV_TIME_BASE/10 < FFMAX(ist->pts, ist->dts)) {
                 ifile->ts_offset -= delta;
                 av_log(NULL, AV_LOG_DEBUG,
-                       "timestamp discontinuity %"PRId64", new offset= %"PRId64"\n",
+                       "timestamp discontinuity for stream #%d:%d "
+                       "(id=%d, type=%s): %"PRId64", new offset= %"PRId64"\n",
+                       ist->file_index, ist->st->index, ist->st->id,
+                       av_get_media_type_string(ist->dec_ctx->codec_type),
                        delta, ifile->ts_offset);
                 pkt.dts -= av_rescale_q(delta, AV_TIME_BASE_Q, ist->st->time_base);
                 if (pkt.pts != AV_NOPTS_VALUE)