void al_color_rgb_to_cmyk(float red, float green, float blue,
float *cyan, float *magenta, float *yellow, float *key)
Each RGB color can be represented in CMYK with a K component of 0 with the following formula:
C = 1 - R M = 1 - G Y = 1 - B K = 0
This function will instead find the representation with the maximal value for K and minimal color components.