/// <summary>
 /// Adds two triangles forming a quad to the MeshCreator. Adds the vertices
 /// to the MeshCreator if they aren't already added.
 /// </summary>
 /// <param name="a">The first vertex in clockwise order.</param>
 /// <param name="b">The second vertex in clockwise order.</param>
 /// <param name="c">The third vertex in clockwise order.</param>
 /// <param name="d">The fourth vertex in clockwise order.</param>
 public void AddQuad(MeshVertex a, MeshVertex b, MeshVertex c, MeshVertex d)
 {
     AddTriangle(a, b, c);
     AddTriangle(a, c, d);
 }