vf_interlace: implement frame rate

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from ffmpeg commit 227b4458)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavfilter/vf_interlace.c

@@ -109,8 +109,10 @@ static int config_out_props(AVFilterLink *outlink)
     outlink->w = inlink->w;
     outlink->h = inlink->h;
     outlink->time_base = inlink->time_base;
+    outlink->frame_rate = inlink->frame_rate;
     // half framerate
     outlink->time_base.num *= 2;
+    outlink->frame_rate.den *= 2;
 
 
     if (s->lowpass) {