per picture psnr

Originally committed as revision 3465 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/snow.c

@@ -2813,7 +2813,7 @@ redo_frame:
                 }
             }
             s->avctx->error[plane_index] += error;
-            s->avctx->error[3] += error;
+            s->current_picture.error[plane_index] = error;
         }
     }