ffmpeg: replace magic number with VSYNC_CFR

ffmpeg.c

@@ -803,7 +803,7 @@ static void do_video_out(AVFormatContext *s,
 
     format_video_sync = video_sync_method;
     if (format_video_sync == VSYNC_AUTO)
-        format_video_sync = (s->oformat->flags & AVFMT_VARIABLE_FPS) ? ((s->oformat->flags & AVFMT_NOTIMESTAMPS) ? VSYNC_PASSTHROUGH : VSYNC_VFR) : 1;
+        format_video_sync = (s->oformat->flags & AVFMT_VARIABLE_FPS) ? ((s->oformat->flags & AVFMT_NOTIMESTAMPS) ? VSYNC_PASSTHROUGH : VSYNC_VFR) : VSYNC_CFR;
 
     switch (format_video_sync) {
     case VSYNC_CFR: