Force backend update when hollowing gizmo is closed

Otherwise, moving holes will not invalidate the drilled mesh
One cannot do it immediately though, because that would also hide the hollowed mesh

src/slic3r/GUI/Gizmos/GLGizmoHollow.cpp

@@ -992,6 +992,7 @@ void GLGizmoHollow::on_set_state()
     }
     if (m_state == Off && m_old_state != Off) { // the gizmo was just turned Off
         //Plater::TakeSnapshot snapshot(wxGetApp().plater(), _(L("SLA gizmo turned off")));
+        m_parent.post_event(SimpleEvent(EVT_GLCANVAS_FORCE_UPDATE));
         m_parent.toggle_model_objects_visibility(true);
         m_c->stash_clipping_plane();
         m_c->m_clipping_plane_distance = 0.f;