ffmpeg: use actual packet pts when discarding for the copypriorss option

The ist->pts value at this point does not necessarily match the actual packet
pts, which is what should be used to decide whether to discard the packet.
Without this change, some video packets that have pts on or after the start
time are getting discarded when they should not be.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

ffmpeg.c

@@ -1388,6 +1388,7 @@ static void do_streamcopy(InputStream *ist, OutputStream *ost, const AVPacket *p
 {
     OutputFile *of = output_files[ost->file_index];
     int64_t ost_tb_start_time = av_rescale_q(of->start_time, AV_TIME_BASE_Q, ost->st->time_base);
+    int64_t ist_tb_start_time = av_rescale_q(of->start_time, AV_TIME_BASE_Q, ist->st->time_base);
     AVPicture pict;
     AVPacket opkt;
 
@@ -1397,9 +1398,15 @@ static void do_streamcopy(InputStream *ist, OutputStream *ost, const AVPacket *p
         !ost->copy_initial_nonkeyframes)
         return;
 
-    if (!ost->frame_number && ist->pts < of->start_time &&
-        !ost->copy_prior_start)
-        return;
+    if (pkt->pts == AV_NOPTS_VALUE) {
+        if (!ost->frame_number && ist->pts < of->start_time &&
+            !ost->copy_prior_start)
+            return;
+    } else {
+        if (!ost->frame_number && pkt->pts < ist_tb_start_time &&
+            !ost->copy_prior_start)
+            return;
+    }
 
     if (of->recording_time != INT64_MAX &&
         ist->pts >= of->recording_time + of->start_time) {