/// <summary>
 /// Calculates the cross-product, but only of the Z component,
 /// because that's all we need in order to do backface culling.
 /// </summary>
 public static float CrossZ(ref Vector3ForCulling left, ref Vector3ForCulling right)
 {
     return (left.X * right.Y) - (left.Y * right.X);
 }
 /// <summary>
 /// Calculates the cross-product, but only of the Z component,
 /// because that's all we need in order to do backface culling.
 /// </summary>
 public static float CrossZ(ref Vector3ForCulling left, ref Vector3ForCulling right)
 {
     return((left.X * right.Y) - (left.Y * right.X));
 }