message: Don't default to ambient=white w/ path tracing
diff: @@ -489,7 +489,7 @@ void renderer::on_display()
  auto epsilon = std::max( 1E-3f, length(diagonal) * 1E-5f );
  auto amb = ambient.x >= 0.0f // if set via cmdline
  ? vec4(ambient, 1.0f)
  - : algo == Pathtracing ? vec4(1.0) : vec4(0.0)
  + : vec4(0.0)
  ;
  if (rt.mode() == host_device_rt::CPU)
  