Attenuation
For real-world lights, the intensity of light decreases as distance from the light
increases, a phenomenon known as attenuation. Since a directional light is
infinitely far away, it doesn't make sense to attenuate its intensity over distance,
so attenuation is disabled for a directional light. However, you might want to
attenuate the light from a positional (point or spot) light. To do this, you can
define an attenuation factor to your light. (OpenGL has three different factors for
attenuation. Here, we just look at the constant attenuation factor).