In volume segment, the minimal angle formed by the bounding cone axis
and the vector pointing from the cluster centroid and any point on the
ray is computed via `dot(bcone.axis, point_to_centroid)`, see Fig.8. in
paper.
For distant light this angle is 0, but due to numerical issues this is
not always true. Therefore explicitly assign `-bcone.axis` to
`point_to_centroid` in this case.