message: Use height-correlated Smith masking-shadowing
  This changelist updates the GLSL implementation of Smith masking-shadowing from uncorrelated to height-correlated forms, as described in  As a future improvement, we should also update from isotropic to anisotropic forms, as described in the same paper.
diff: @@ -42,18 +42,14 @@ vec3 mx_microfacet_ggx_IS(vec2 Xi, vec3 X, vec3 Y, vec3 N, float alphaX, float a
  return normalize(H);
  }
  -// https://www.cs.cornell.edu/~srm/publications/EGSR07-btdf.pdf (Equation 34)
  -float mx_microfacet_ggx_G1(float cosTheta, float alpha)
  -{
  - float cosTheta2 = cosTheta * cosTheta;
  - float tanTheta2 = (1.0 - cosTheta2) / cosTheta2;
  - return 2.0 / (1.0 + sqrt(1.0 + alpha * alpha * tanTheta2));
  -}
  -
  -// https://www.cs.cornell.edu/~srm/publications/EGSR07-btdf.pdf (Equation 23)
  +// http://jcgt.org/published/0003/02/03/paper.pdf
  +// Equations 72 and 99
  float mx_microfacet_ggx_smith_G(float NdotL, float NdotV, float alpha)
  {
  - return mx_microfacet_ggx_G1(NdotL, alpha) * mx_microfacet_ggx_G1(NdotV, alpha);
  + float alpha2 = mx_square(alpha);
  + float lambdaL = sqrt(alpha2 + (1.0 - alpha2) * mx_square(NdotL));
  + float lambdaV = sqrt(alpha2 + (1.0 - alpha2) * mx_square(NdotV));
  + return 2.0 / (lambdaL / NdotL + lambdaV / NdotV);
  }
  // http://blog.selfshadow.com/publications/s2017-shading-course/imageworks/s2017_pbs_imageworks_sheen.pdf (Equation 2)
  