message: improve quality of diffuse sampling on desktop
  We 4-tap filter the diffuse map, which generally is very small
  (e.g. 16x16), to improve quality a bit.
diff: @@ -72,7 +72,23 @@ vec3 Irradiance_RoughnessOne(const vec3 n) {
  vec3 diffuseIrradiance(const vec3 n) {
  if (frameUniforms.iblSH[0].x == 65504.0) {
  +#if defined(TARGET_MOBILE)
  return Irradiance_RoughnessOne(n);
  +#else
  + ivec2 s = textureSize(light_iblSpecular, int(frameUniforms.iblRoughnessOneLevel));
  + float du = 1.0 / float(s.x);
  + float dv = 1.0 / float(s.y);
  + vec3 m0 = normalize(cross(n, vec3(0.0, 1.0, 0.0)));
  + vec3 m1 = cross(m0, n);
  + vec3 m0du = m0 * du;
  + vec3 m1dv = m1 * dv;
  + vec3 c;
  + c = Irradiance_RoughnessOne(n - m0du - m1dv);
  + c += Irradiance_RoughnessOne(n + m0du - m1dv);
  + c += Irradiance_RoughnessOne(n + m0du + m1dv);
  + c += Irradiance_RoughnessOne(n - m0du + m1dv);
  + return c * 0.25;
  +#endif
  } else {
  return Irradiance_SphericalHarmonics(n);
  }
  