Reference geometric normal in mx_forward_facing_normal

[jstone-lucasfilm]

As of today, our GLSL implementation of mx_forward_facing_normal uses the per-pixel shading normal to compute whether a surface is forward or back-facing, flipping that same per-pixel normal when the surface is found to be back-facing:

MaterialX/libraries/pbrlib/genglsl/lib/mx_microfacet.glsl

Line 56 in d114ab7

vec3 mx_forward_facing_normal(vec3 N, vec3 V)

Ideally, we should be using the geometric normal to test the facing of the surface, flipping the per-pixel shading normal when the dot product between the geometric normal and view direction is negative. See the OSL codebase for a good reference of this approach:

https://github.com/AcademySoftwareFoundation/OpenShadingLanguage/blob/71dd75945f5257f00ecac1fe151901e81410680b/src/shaders/stdosl.h#L204