message: check for NaN samples in delta tracking, closes
diff: @@ -48,8 +48,12 @@ float delta_tracking(VolumetricModel *uniform vModel,
  xi = RandomSampler_getFloat(randomSampler);
  const vec3f p = o + t * w;
  +
  const float sample = vklComputeSampleV(
  vModel->volume->vklSampler, (const varying vkl_vec3f *uniform) & p);
  + if (isnan(sample))
  + continue;
  +
  const vec4f color =
  vModel->transferFunction->get(vModel->transferFunction, sample);
  mu_t = vModel->densityScale * color.w;
  