materials: use crate definition for BLACK.

rtchallenge/src/materials.rs

@@ -2,6 +2,7 @@ use crate::{
     lights::PointLight,
     tuples::Color,
     tuples::{dot, reflect, Tuple},
+    BLACK,
 };
 #[derive(Debug, PartialEq, Clone)]
 pub struct Material {
@@ -42,8 +43,6 @@ impl Default for Material {
     }
 }
 
-const BLACK: Color = Color::new(0., 0., 0.);
-
 /// Compute lighting contributions using the Phong reflection model.
 ///
 /// # Examples