private void ExportCollada_Material(XmlWriter xml, Kn5Material material) {
xml.WriteStartElement("material");
xml.WriteAttributeStringSafe("id", $"{material.Name}-material");
xml.WriteAttributeStringSafe("name", material.Name);
xml.WriteStartElement("instance_effect");
xml.WriteAttributeStringSafe("url", $"#{material.Name}-effect");
xml.WriteEndElement();
xml.WriteEndElement();
}