wip OSL reflection / refraction.
Specular BSDFs working in OSL.
Merge remote-tracking branch 'upstream/master' into OSL_specular_bsdfs
Added more comments.
Added reference image for simple glass scene.
Optimized the fresnel multiplier == 0 case. Removed some unused files.
Final cleanup.
Fixed a bug in refraction closure. Updated test ref image.
Small improvements to simple glass shader.
Removed unused include
Fixed PR comments.
Merge pull request #448 from est77/OSL_specular_bsdfs

OSL specular bsdfs
Made fresnel multiplier param texturable.
Merge pull request #451 from est77/master

Made fresnel multiplier param of SpecularBTDF texturable.
Made Material a real polymorphic class.
Small formatting fix.
Merge pull request #452 from est77/polymorphic_materials

Made Material a real polymorphic class.
small code cleanups.
added foundation::binomial().
made foundation::binomial() inline.