avfilter/vf_fps: Avoid inlink fifo build up.

When duplicating frames we need to schedule for activation again, otherwise frames can build up in the inlink fifo.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_fps.c

@@ -256,7 +256,7 @@ static int write_frame(AVFilterContext *ctx, FPSContext *s, AVFilterLink *outlin
         av_log(ctx, AV_LOG_DEBUG, "Writing frame with pts %"PRId64" to pts %"PRId64"\n",
                s->frames[0]->pts, frame->pts);
         s->cur_frame_out++;
-
+        *again = 1;
         return ff_filter_frame(outlink, frame);
     }
 }