Reference geometric normal in `mx_forward_facing_normal`

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:

https://github.com/AcademySoftwareFoundation/MaterialX/blob/d114ab795c0ad780b2dfce0c2b6ca3b4993dff65/libraries/pbrlib/genglsl/lib/mx_microfacet.glsl#L56

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

Start in libraries/pbrlib/genglsl/lib/mx_microfacet.glsl at the referenced mx_forward_facing_normal implementation, then compare the facing logic with src/shaders/stdosl.h in OpenShadingLanguage. Confirm the geometric normal controls the facing test while the shading normal is the one flipped, and verify the generated shader behavior for front- and back-facing surfaces.

Written by the indexing model from the issue text.