libgimp: in GimpTileBackendPlugin, change default tile multiplier to 1

In GimpTileBackendPlugin, change the default tile multiplier,
specifying the ratio between the backend tile-size, and GIMP's
tile-size, from 2 to 1.  Since we're reading/writing each GIMP tile
using a separate command anyway, using a large multiplier doesn't
provide any benefits, while it does have drawbacks.  In particular,
it reduces the chance that a write operation will affect an entire
tile, which allows us to avoid reading the tile data from GIMP.

(cherry picked from commit a5e2945b68)

libgimp/gimptilebackendplugin.c

@@ -43,7 +43,7 @@ struct _GimpTileBackendPluginPrivate
 static gint
 gimp_gegl_tile_mul (void)
 {
-  static gint     mul    = 2;
+  static gint     mul    = 1;
   static gboolean inited = FALSE;
 
   if (G_LIKELY (inited))