message: For Debug EyeLight renderer, flip backfacing normals
diff: @@ -28,7 +28,7 @@ bool hitBackground(
  float eyeLight(varying ScreenSample &sample)
  {
  - return 0.2f + 0.8f * dot(normalize(sample.ray.Ng), neg(sample.ray.dir));
  + return 0.2f + 0.8f * abs(dot(normalize(sample.ray.Ng), sample.ray.dir));
  }
  /* a simple test-frame renderer that doesn't even trace a ray, just
  