In gimp_group_layer_update_size(), never suspend drawable updates
(and, in fact, remove the option to suspend drawable updates
entirely,) and instead never update the drawable during the call to
gimp_drawable_set_buffer_full(), and flush the group's projection
*after* setting the drawable's buffer, so that any pending updates
will happen after the group's buffer and size are up-to-date.

This fixes some missed drawable updates.