message: Russian roulette
diff: @@ -147,6 +147,12 @@ struct kernel
  throughput = mul( throughput, src, !zero_pdf, throughput );
  throughput = select( zero_pdf, C(0.0), throughput );
  + // Russian roulette
  + auto prob = max_element(throughput.samples());
  + auto terminate = gen.next() > prob;
  + active_rays &= !terminate;
  + throughput /= prob;
  +
  if (!any(active_rays))
  {
  break;
  