a zero duration will lead to a non-monotonic non-incrementing pts, so we have to use 1 instead.

src/zm_videostore.cpp

@@ -722,7 +722,7 @@ int VideoStore::writeVideoFramePacket(AVPacket *ipkt) {
         ipkt->pts,
         video_last_pts,
         duration);
-    if (duration < 0) {
+    if (duration <= 0) {
       duration = ipkt->duration ? ipkt->duration : av_rescale_q(1,video_in_stream->time_base, video_out_stream->time_base);
     }
   }