The Rest: to_light.make_unit_vector is not defined anywhere

[D-K-E]

The Rest: listing 20 contains a method make_unit_vector which is not defined anywhere.
Probably unit_vector function is intended from vec3.h.
The problem persists in dev-major/minor/patch branches as well.

[hollasch]

Messed up and made correction directly on dev-patch (no PR). Corrected in e46df59