app: force the image flush after a selection fill.

Other bucket fills are now done as filter until committed, but basic
selection fill is still done automatically. So let's make sure the
canvas is updated immediately (as it used to be before my changes).

app/tools/gimpbucketfilltool.c

@@ -511,6 +511,7 @@ gimp_bucket_fill_tool_button_press (GimpTool            *tool,
           if (options->fill_selection)
             {
               gimp_drawable_edit_fill (drawable, fill_options, NULL);
+              gimp_image_flush (image);
             }
           else
             {