Reviewers: osl-dev_googlegroups.com, dev-osl <dev-osl_imageworks.com>,

Description:
And also adding the backfacing() function for the shaders to be able to
handle that directly.

Please review this at http://codereview.appspot.com/1854047/show

Affected files:
src/include/oslclosure.h
src/include/oslexec.h
src/liboslcomp/typecheck.cpp
src/liboslexec/bsdf_ashikhmin_velvet.cpp
src/liboslexec/bsdf_cloth.cpp
src/liboslexec/bsdf_cloth_specular.cpp
src/liboslexec/bsdf_diffuse.cpp
src/liboslexec/bsdf_fakefur.cpp
src/liboslexec/bsdf_hair.cpp
src/liboslexec/bsdf_microfacet.cpp
src/liboslexec/bsdf_phong.cpp
src/liboslexec/bsdf_reflection.cpp
src/liboslexec/bsdf_refraction.cpp
src/liboslexec/bsdf_transparent.cpp
src/liboslexec/bsdf_ward.cpp
src/liboslexec/bsdf_westin.cpp
src/liboslexec/builtin_closures.cpp
src/liboslexec/closure_test.cpp
src/liboslexec/context.cpp
src/liboslexec/emissive.cpp
src/liboslexec/llvm_instance.cpp
src/liboslexec/llvm_ops.cpp
src/liboslexec/master.cpp
src/liboslexec/opattribute.cpp
src/liboslexec/opclosure.cpp
src/liboslexec/oslexec_pvt.h
src/liboslexec/oslops.h
src/liboslexec/shadingsys.cpp

LGTM

http://codereview.appspot.com/1854047/show