libgimp: in GimpTileBackendPlugin, don't read tile data upon TILE_SET

Add an internal _gimp_tile_ref_noinit() function, which increases
the ref-count of a tile *without* initializing its data (in
particular, without reading its data from GIMP, or zeroing it.)
Use this function, instead of gimp_tile_ref(), when storing a tile
in GimpTileBackendPlugin, to avoid unnecessarily reading the tile
data from GIMP.

libgimp/gimptilebackendplugin.c

@@ -261,7 +261,7 @@ gimp_tile_write_mul (GimpTileBackendPlugin *backend_plugin,
           gimp_tile = gimp_drawable_get_tile (priv->drawable,
                                               priv->shadow,
                                               y+v, x+u);
-          gimp_tile_ref (gimp_tile);
+          _gimp_tile_ref_noinit (gimp_tile);
 
           {
             gint ewidth           = gimp_tile->ewidth;