lavfi/avf_showcqt: set frame durations

The filter is supposed to produce CFR output.

libavfilter/avf_showcqt.c

@@ -1187,6 +1187,7 @@ static int plot_cqt(AVFilterContext *ctx, AVFrame **frameout)
             UPDATE_TIME(s->sono_time);
         }
         out->pts = s->next_pts;
+        out->duration = PTS_STEP;
         s->next_pts += PTS_STEP;
     }
     s->sono_count = (s->sono_count + 1) % s->count;