A cone is represented by its location, axis vector, and half-angle. The
reference direction need not be perpendicular to the axis vector but must
not be parallel to it. Parameterization is 0 to 2pi in u and length along
the axis in v, positive in the direction of the axis vector. The point
defines the apex of the cone. A mathematical cone consists of two half-cones.
The half retained by this representation is the half in which the axis points
away from the apex of the cone. Note that this means that v parameter values
are always positive.