message: Fix numerical warnings in GLSL
  This changelist adds a guard to the GLSL implementation of thin-film rendering, addressing numerical warnings and visual artifacts in situations where eta values decrease between interfaces.  Without this guard, the logic requires the computation of an imaginary square root, which is undefined behavior in GLSL.
diff: @@ -348,7 +348,7 @@ vec3 mx_fresnel_airy(float cosTheta, vec3 ior, vec3 extinction, float tf_thickne
  // Assume vacuum on the outside
  float eta1 = 1.0;
  - float eta2 = tf_ior;
  + float eta2 = max(tf_ior, eta1);
  // Optical path difference
  float cosTheta2 = sqrt(1.0 - mx_square(eta1/eta2) * (1.0 - mx_square(cosTheta)));
  