Fix: hit record selection mechanism

Because of the mutable record being used in the loop, the previous
version had a somewhat obscured way to track the nearest collision.

Switching to an optional (so I can have a non-optional Material in it)
means I'm not interrogating that value.... So it gets to be explicit
again.

I'll refactor the entire for-loop into an iterator with the min()
adapter at some point. For now: Material lifetimes!

src/scene.rs

@@ -32,10 +32,11 @@ impl Hittable {
             Hittable::HittableList { hittables } => {
                 let mut might_return: Option<HitRecord> = None;
                 let mut hit_anything = false;
-
+                let mut nearest_t = t_max;
                 for item in hittables {
-                    if let Some(record) = item.hit(r, t_min, t_max){
+                    if let Some(record) = item.hit(r, t_min, nearest_t){
                         hit_anything = true;
+                        nearest_t = record.t;
                         might_return = Some(record);
                     }
                 }