"To specify the angle between the axis of the cone an a ray along the
edge of the cone, use the GL_SPOT_CUTOFF parameter….By default, the
spotlight feature is disabled because the GL_SPOT_CUTOFF parameter is
180.0. This value means that light is emitted in all directions (the
angle at the cone’s apex is 360 degrees,so it isn’t a cone at all). The
value for GL_SPOT_CUTOFF is restricted to the range [0.0 90.0] (unless
it has the special value 180.)."