Forgot a `facing` component

[mysteryDate]

From http://developer.download.nvidia.com/CgTutorial/cg_tutorial_chapter05.html:

Additionally, the specular term is forced to zero if the diffuse term is zero because N · L (from diffuse lighting) is negative. This ensures that specular highlights do not appear on geometry that faces away from the light.

So line 9 should read:

return pow(step(0.0, dot(surfaceNormal, lightDirection)) * max(0.0, dot(viewDirection, R)), shininess);

Or something like that.