fixbug: save mtl file StreamWriter has no close

Author: huqingjie

src/SharpGLTF.Toolkit/IO/WavefrontWriter.cs

@@ -163,6 +163,7 @@ private static Dictionary<Material, string> _GetMaterialFileGenerator(IDictionar
 
                     sw.WriteLine();
                 }
+                sw.Close();
             };
 
             return mmap;