ft: 5,6.1: sphere class + normals

src/ray.rs

@@ -6,7 +6,7 @@ pub struct Ray {
 }
 
 impl Ray {
-    pub fn at(self, t: f32) -> Vec3 {
+    pub fn at(&self, t: f32) -> Vec3 {
         self.origin + t * self.dir
     }