movenc: Write pts timestamps in tfxd, instead of dts timestamps

This matches what we write in tfra and tfrf since 9cbf70fa.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.c

@@ -2542,7 +2542,8 @@ static int mov_write_tfxd_tag(AVIOContext *pb, MOVTrack *track)
     avio_write(pb, uuid, sizeof(uuid));
     avio_w8(pb, 1);
     avio_wb24(pb, 0);
-    avio_wb64(pb, track->frag_start);
+    avio_wb64(pb, track->start_dts + track->frag_start +
+                  track->cluster[0].cts);
     avio_wb64(pb, track->start_dts + track->track_duration -
                   track->cluster[0].dts);