For performance reasons the scope of that library is limited to "simple" adjustements that can be performed directly on RGB values.  Adjusting saturation would require converting to HSB and back to RGB per pixel, so not implemented.

But you could write your own - just loop through the pixels[] array, retrieve the color, retrieve the HSB values, modify S (and/or H, B) as desired, recreate an RGB color from your modified values, stuff it back into the pixels array.

Cheers,