vf_boxblur: call avfilter_draw_slice() at the end of draw_slice()

libavfilter/vf_boxblur.c

@@ -328,6 +328,8 @@ static void draw_slice(AVFilterLink *inlink, int y0, int h0, int slice_dir)
               outpicref->data[plane], outpicref->linesize[plane],
               w[plane], h[plane], boxblur->radius[plane], boxblur->power[plane],
               boxblur->temp);
+
+    avfilter_draw_slice(outlink, y0, h0, slice_dir);
 }
 
 AVFilter avfilter_vf_boxblur = {