ffmpeg: Use input packet duration in vfr/cfr code if available and valid

Fixes Ticket3052

ffmpeg.c

@@ -893,6 +893,14 @@ static void do_video_out(AVFormatContext *s,
     if(ist && ist->st->start_time != AV_NOPTS_VALUE && ist->st->first_dts != AV_NOPTS_VALUE && ost->frame_rate.num)
         duration = 1/(av_q2d(ost->frame_rate) * av_q2d(enc->time_base));
 
+    if (!ost->filters_script &&
+        !ost->filters &&
+        next_picture &&
+        ist &&
+        lrintf(av_frame_get_pkt_duration(next_picture) * av_q2d(ist->st->time_base) / av_q2d(enc->time_base)) > 0) {
+        duration = lrintf(av_frame_get_pkt_duration(next_picture) * av_q2d(ist->st->time_base) / av_q2d(enc->time_base));
+    }
+
     sync_ipts = next_picture->pts;
     delta0 = sync_ipts - ost->sync_opts;
     delta  = delta0 + duration;

tests/ref/fate/gif-disposal-restore

 #tb 0: 1/1
 0,          0,          0,        1,   112320, 0xb8afe429
+0,          0,          0,        1,   112320, 0xae588a4b
 0,          3,          3,        1,   112320, 0xccdd27b7