Do not duplicate frames if the muxer claims that it is not needed.

Note to release manager, this is risky and should be reverted if
it causes any problem.

Originally committed as revision 17555 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -865,7 +865,7 @@ static void do_video_out(AVFormatContext *s,
         //FIXME set to 0.5 after we fix some dts/pts bugs like in avidec.c
         if (vdelta < -1.1)
             nb_frames = 0;
-        else if (video_sync_method == 2)
+        else if (video_sync_method == 2 || (video_sync_method<0 && (s->oformat->flags & AVFMT_VARIABLE_FPS)))
             ost->sync_opts= lrintf(get_sync_ipts(ost) / av_q2d(enc->time_base));
         else if (vdelta > 1.1)
             nb_frames = lrintf(vdelta);