Fix G3djModelWriter - nodes/parts instead of nodes/materials

G3djModelLoader expects "parts" while fbx-conv wrote "materials" inside node list.


		Array<OrderedMap<String, Object>> materials = (Array<OrderedMap<String, Object>>)json.get("parts");
		if(materials != null){
			jsonNode.parts = new ModelNodePart[materials.size];
			
			int i = 0;
			for(OrderedMap<String, Object> material : materials) {

I'm not sure which one (conv or loader) implementation is correct. Wither exporter or importer should be fixed.
"parts" looks more suitable.