copy method
Implementation
copy(source) {
super.copy(source);
var _source = source as GLTFMeshStandardSGMaterial;
this.specularMap = _source.specularMap;
this.specular!.copy(_source.specular!);
this.glossinessMap = _source.glossinessMap;
this.glossiness = _source.glossiness;
// delete this.metalness;
// delete this.roughness;
// delete this.metalnessMap;
// delete this.roughnessMap;
return this;
}