fftools/ffmpeg: move stream-dependent starttime correction to transcode_init()

Currently this code is located in the discontinuity handling block,
where it does not belong.

fftools/ffmpeg.c

@@ -3313,6 +3313,28 @@ static int transcode_init(void)
                 input_streams[j + ifile->ist_index]->start = av_gettime_relative();
     }
 
+    // Correct starttime based on the enabled streams
+    for (i = 0; i < nb_input_files; i++) {
+        InputFile       *ifile = input_files[i];
+        AVFormatContext    *is = ifile->ctx;
+        int64_t new_start_time = INT64_MAX;
+
+        if (is->start_time == AV_NOPTS_VALUE ||
+            !(is->iformat->flags & AVFMT_TS_DISCONT))
+            continue;
+
+        for (int j = 0; j < is->nb_streams; j++) {
+            AVStream *st = is->streams[j];
+            if(st->discard == AVDISCARD_ALL || st->start_time == AV_NOPTS_VALUE)
+                continue;
+            new_start_time = FFMIN(new_start_time, av_rescale_q(st->start_time, st->time_base, AV_TIME_BASE_Q));
+        }
+        if (new_start_time > is->start_time) {
+            av_log(is, AV_LOG_VERBOSE, "Correcting start time by %"PRId64"\n", new_start_time - is->start_time);
+            ifile->ts_offset = -new_start_time;
+        }
+    }
+
     /* init input streams */
     for (i = 0; i < nb_input_streams; i++)
         if ((ret = init_input_stream(i, error, sizeof(error))) < 0)
@@ -3752,24 +3774,6 @@ static int process_input(int file_index)
 
     if(!ist->wrap_correction_done && is->start_time != AV_NOPTS_VALUE && ist->st->pts_wrap_bits < 64){
         int64_t stime, stime2;
-        // Correcting starttime based on the enabled streams
-        // FIXME this ideally should be done before the first use of starttime but we do not know which are the enabled streams at that point.
-        //       so we instead do it here as part of discontinuity handling
-        if (   ist->next_dts == AV_NOPTS_VALUE
-            && ifile->ts_offset == -is->start_time
-            && (is->iformat->flags & AVFMT_TS_DISCONT)) {
-            int64_t new_start_time = INT64_MAX;
-            for (i=0; i<is->nb_streams; i++) {
-                AVStream *st = is->streams[i];
-                if(st->discard == AVDISCARD_ALL || st->start_time == AV_NOPTS_VALUE)
-                    continue;
-                new_start_time = FFMIN(new_start_time, av_rescale_q(st->start_time, st->time_base, AV_TIME_BASE_Q));
-            }
-            if (new_start_time > is->start_time) {
-                av_log(is, AV_LOG_VERBOSE, "Correcting start time by %"PRId64"\n", new_start_time - is->start_time);
-                ifile->ts_offset = -new_start_time;
-            }
-        }
 
         stime = av_rescale_q(is->start_time, AV_TIME_BASE_Q, ist->st->time_base);
         stime2= stime + (1ULL<<ist->st->pts_wrap_bits);