world: don't normalize light brightness by quantity of lights.

That's not how they work in real life.

rtchallenge/src/world.rs

@@ -6,7 +6,7 @@ use crate::{
     rays::Ray,
     shapes::{intersect, Shape},
     tuples::{Color, Tuple},
-    Float, BLACK, WHITE,
+    BLACK, WHITE,
 };
 
 /// World holds all drawable objects and the light(s) that illuminate them.
@@ -145,7 +145,7 @@ impl World {
                     shadowed,
                 )
             });
-        c / self.lights.len() as Float
+        c
     }
     /// Compute color for given ray fired at the world.
     ///