app: fix update of group layers when size changes only trigger an offset change

In gimp_group_layer_update_size(), when the new size happens to be the
same as the old size, we need to call gimp_pickable_flush() on the
group projection so it is ready to be used for rendering the image,
just like in gimp_group_layer_stack_update().

(cherry picked from commit 2f8476bd)