Restore original mx_environment_radiance

This changelist restores the original implementation of mx_environment_radiance in the prefiltered path, as there does not appear to be any visual difference when switching between the original and new implementations.

libraries/pbrlib/genglsl/lib/mx_environment_prefilter.glsl

@@ -15,18 +15,12 @@ vec3 mx_environment_radiance(vec3 N, vec3 V, vec3 X, vec2 alpha, int distributio
  float NdotV = clamp(dot(N, V), M_FLOAT_EPS, 1.0);
 
  float avgAlpha = mx_average_alpha(alpha);
- // Convert n and k to f0. f90 will be equal to 1.
- // Source: Section 2.2: https://jcgt.org/published/0003/04/03/paper.pdf
- vec3 k2 = fd.extinction * fd.extinction;
- vec3 F0 = ((fd.ior - 1)*(fd.ior - 1) + k2) / ((fd.ior + 1)*(fd.ior + 1) + k2);
- vec3 FGD = mx_ggx_dir_albedo(NdotV, avgAlpha, F0, vec3(1.0));
- if (fd.refraction) {
- FGD = 1.0 - FGD;
- }
-
+ vec3 F = mx_compute_fresnel(NdotV, fd);
+ float G = mx_ggx_smith_G2(NdotV, NdotV, avgAlpha);
+ vec3 FG = fd.refraction ? vec3(1.0) - (F * G) : F * G;
 
  vec3 Li = mx_latlong_map_lookup(L, $envMatrix, mx_latlong_compute_lod(avgAlpha), $envRadiance);
- return Li * FGD;
+ return Li * FG;
 }
 
 vec3 mx_environment_irradiance(vec3 N)