message: Fix initialization warning in GLSL
  This changelist refactors the implementation of mx_surface_transmission in GLSL, addressing an initialization warning when the function is called in WebGL.
diff: vec3 mx_surface_transmission(vec3 N, vec3 V, vec3 X, vec2 alpha, int distribution, FresnelData fd)
  {
  + vec3 transmission;
  if ($refractionEnv)
  {
  - // Approximate the appearance of dielectric transmission as glossy
  + // Approximate the appearance of surface transmission as glossy
  // environment map refraction, ignoring any scene geometry that might
  // be visible through the surface.
  fd.refraction = true;
  - return mx_environment_radiance(N, V, X, alpha, distribution, fd);
  + transmission = mx_environment_radiance(N, V, X, alpha, distribution, fd);
  + }
  + else
  + {
  + transmission = $refractionColor;
  }
  - return $refractionColor;
  + return transmission;
  }
  