some refactoring and proper DoF support

src/sphere.rs

@@ -8,12 +8,12 @@ use crate::Ray;
 pub struct Sphere {
     center: Point3,
     radius: f64,
-    mat_ptr: Box<dyn Material>,
+    material: Box<dyn Material>,
 }
 
 impl Sphere {
-    pub fn new<T: Material + 'static>(center: Point3, radius: f64, mat_ptr: Box<T>) -> Self {
-        Sphere {center, radius, mat_ptr}
+    pub fn new<T: Material + 'static>(center: Point3, radius: f64, material: Box<T>) -> Self {
+        Sphere {center, radius, material}
     }
 }
 
@@ -54,7 +54,7 @@ impl Hittable for Sphere {
             p: p,
             normal: outward_normal,
             front_face,
-            mat_ptr: self.mat_ptr.deref(),
+            material: self.material.deref(),
         })
     }
 }