This function returns a color.
The color is the sum of the diffuse reflection for all the light sources
multiplied by the co-efficient Kd and the surface color DIF.

This must be a float.  Defaults to 1.0.
This is the co-efficient for the diffuse function.
It controls the intensity of the returned color.
For best results Kd should range from [0,1].

This must be a color. It is the surface color.
It tints the diffuse contribution from the lights.

This must be a vector. The default is state->normal.
This is the normal to the surface point to be shaded.

This function returns a color.
The color is the sum of the diffuse reflection for all the light sources
multiplied by the co-efficient Kd and the surface color DIF.