message: Minor fixes to mx_fresnel_airy in GLSL
  Removed unused variable R21
  Updated link to Laurent Belcour's paper
diff: @@ -340,7 +340,7 @@ vec3 mx_eval_sensitivity(float opd, float shift)
  }
  // A Practical Extension to Microfacet Theory for the Modeling of Varying Iridescence
  -// https://belcour.github.io/blog/research/2017/05/01/brdf-thin-film.html
  +// https://belcour.github.io/blog/research/publication/2017/05/01/brdf-thin-film.html
  vec3 mx_fresnel_airy(float cosTheta, vec3 ior, vec3 extinction, float tf_thickness, float tf_ior)
  {
  // Convert nm -> m
  @@ -357,7 +357,6 @@ vec3 mx_fresnel_airy(float cosTheta, vec3 ior, vec3 extinction, float tf_thickne
  // First interface
  vec2 R12, phi12;
  mx_fresnel_dielectric_polarized(cosTheta, eta1, eta2, R12, phi12);
  - vec2 R21 = R12;
  vec2 T121 = vec2(1.0) - R12;
  vec2 phi21 = vec2(M_PI) - phi12;
  