Fix DisneyBSSRDF::S().
Fix DisneyBSSRDF::S().

Sw() was adding an extra Fresnel term and an unneeded normalization
scale.

Issue #173

nyue/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3