void storeLight(
        tinyxml2::XMLElement& elt,
        const DirectionalLight& light) {
    storeVector(elt, light.m_IncidentDirection);
    storeColor(elt, light.m_Le);
    elt.SetName("DirectionalLight");
}
void storeLight(
        tinyxml2::XMLElement& elt,
        const PointLight& light) {
    storeVector(elt, light.m_Position);
    storeColor(elt, light.m_Intensity);
    elt.SetName("PointLight");
}