per picture psnr

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@3465 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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;
         }
     }