Build a rotation matrix.
        vec3 is a Vector3 defining the axis about which to rotate the object.

def build_rotation(vec3):
        """
        Build a rotation matrix.
        vec3 is a Vector3 defining the axis about which to rotate the object.
        """
        if not isinstance(vec3, Vector3):
            raise ValueError("rotAmt must be a Vector3")
        return Matrix4.x_rotate(vec3.x) * Matrix4.y_rotate(vec3.y) * Matrix4.z_rotate(vec3.z)