fftools/ffmpeg: replace AVFrame.pkt_duration with duration

Mistakenly reintroduced in 4740fea7dd.

fftools/ffmpeg.c

@@ -1216,18 +1216,18 @@ static void do_video_out(OutputFile *of,
                 if (delta0 > 1.1)
                     nb0_frames = llrintf(delta0 - 0.6);
             }
-            next_picture->pkt_duration = 1;
+            next_picture->duration = 1;
             break;
         case VSYNC_VFR:
             if (delta <= -0.6)
                 nb_frames = 0;
             else if (delta > 0.6)
                 ost->sync_opts = llrint(sync_ipts);
-            next_picture->pkt_duration = duration;
+            next_picture->duration = duration;
             break;
         case VSYNC_DROP:
         case VSYNC_PASSTHROUGH:
-            next_picture->pkt_duration = duration;
+            next_picture->duration = duration;
             ost->sync_opts = llrint(sync_ipts);
             break;
         default:

fftools/sync_queue.c

@@ -76,7 +76,7 @@ static int64_t frame_ts(const SyncQueue *sq, SyncQueueFrame frame)
 {
     return (sq->type == SYNC_QUEUE_PACKETS) ?
            frame.p->pts + frame.p->duration :
-           frame.f->pts + frame.f->pkt_duration;
+           frame.f->pts + frame.f->duration;
 }
 
 static int frame_null(const SyncQueue *sq, SyncQueueFrame frame)