lavfi: add a frame_rate field to AVFilterLink.

libavfilter/avfilter.c

@@ -243,6 +243,9 @@ int avfilter_config_links(AVFilterContext *filter)
                     link->sample_aspect_ratio = inlink ?
                         inlink->sample_aspect_ratio : (AVRational){1,1};
 
+                if (inlink && !link->frame_rate.num && !link->frame_rate.den)
+                    link->frame_rate = inlink->frame_rate;
+
                 if (inlink) {
                     if (!link->w)
                         link->w = inlink->w;

libavfilter/avfilter.h

@@ -738,6 +738,18 @@ struct AVFilterLink {
      */
     int age_index;
 
+    /**
+     * Frame rate of the stream on the link, or 1/0 if unknown;
+     * if left to 0/0, will be automatically be copied from the first input
+     * of the source filter if it exists.
+     *
+     * Sources should set it to the best estimation of the real frame rate.
+     * Filters should update it if necessary depending on their function.
+     * Sinks can use it to set a default output frame rate.
+     * It is similar to the r_frae_rate field in AVStream.
+     */
+    AVRational frame_rate;
+
 };
 
 /**