Fix incorrect background color handling in glossy reflections.

Bug introduced in the previous refector removing the LightPath struct.

src/renderer.rs

@@ -500,7 +500,8 @@ fn trace_camera_light_path(
                 .background_color
                 .to_spectral_sample(ray.wavelength)
                 .e
-                * mis_pdf;
+                * attenuation
+                / mis_pdf;
 
             break;
         }