lavf: estimate frame duration from r_frame_rate.

If r_frame_rate is set, it should be more reliable for this than either
codec or stream timebase.

libavformat/utils.c

@@ -826,7 +826,10 @@ static void compute_frame_duration(int *pnum, int *pden, AVStream *st,
     *pden = 0;
     switch(st->codec->codec_type) {
     case AVMEDIA_TYPE_VIDEO:
-        if(st->time_base.num*1000LL > st->time_base.den){
+        if (st->r_frame_rate.num) {
+            *pnum = st->r_frame_rate.den;
+            *pden = st->r_frame_rate.num;
+        } else if(st->time_base.num*1000LL > st->time_base.den) {
             *pnum = st->time_base.num;
             *pden = st->time_base.den;
         }else if(st->codec->time_base.num*1000LL > st->codec->time_base.den){