Updated fractal.h for unitialized variable warning in voronoi functions. Added simple shadeop to access glibc's complementary error function, used in the Cook-Torrance shading function, for the He-Torrance geometric attenuation. Updated all shading models in respective header. Updated Cook-Torrance block to add access to multiple microfacet distributions, and geometric attenuation functions, defined in distributions2.h header. Updated shading models header to add new Cook-Torrance model. Improved all other shading models. Removed anisoct block, since it's not part of the new Cook-Torrance block.
