vf_psnr: add psnr_avg to stats file.

libavfilter/vf_psnr.c

@@ -174,6 +174,7 @@ static AVFrame *do_psnr(AVFilterContext *ctx, AVFrame *main,
             c = s->is_rgb ? s->rgba_map[j] : j;
             fprintf(s->stats_file, "mse_%c:%0.2f ", s->comps[j], comp_mse[c]);
         }
+        fprintf(s->stats_file, "psnr_avg:%0.2f ", get_psnr(mse, 1, s->average_max));
         for (j = 0; j < s->nb_components; j++) {
             c = s->is_rgb ? s->rgba_map[j] : j;
             fprintf(s->stats_file, "psnr_%c:%0.2f ", s->comps[j],