avformat/img2dec: set r_frame_rate in addition to avg_frame_rate

Apparently for various image sequences libavformat/utils.c can
calculate rather fancy r_frame_rate values, such as `186/1921`,
and since ffmpeg.c utilizes r_frame_rate for the filter chain
time base, this can quite deteriorate the output frame timing - even
though the user has requested the image sequence to be interpreted
at a specific, constant frame rate.

libavformat/img2dec.c

@@ -222,7 +222,7 @@ int ff_img_read_header(AVFormatContext *s1)
         avpriv_set_pts_info(st, 64, 1, 1);
     else {
         avpriv_set_pts_info(st, 64, s->framerate.den, s->framerate.num);
-        st->avg_frame_rate = s->framerate;
+        st->avg_frame_rate = st->r_frame_rate = s->framerate;
     }
 
     if (s->width && s->height) {