Fix: Nearest root within tolerance

I found the fucker. The rays were bouncing incorrectly because I wasn't
holding a value the way I should have. I thought I was being clever and
using Rusts's ability to redeclare variables. But, no. That value is
meant to be modified when the first conditional passes. The outcomes are
3 possibilities, where one is an early return. Shadowing the value that
  way meant I was giving back garbage.

I don't know why this didn't have any obviously bad effects prior to
making the dielectric material.

src/sphere.rs

@@ -27,9 +27,9 @@ impl Hittable for Sphere {
         let sqrtd = discriminant.sqrt();
         
         // nearest root that lies within tolerance
-        let root = (-half_b - sqrtd) / a;
+        let mut root = (-half_b - sqrtd) / a;
         if root < t_min || root > t_max {
-            let root = (-half_b + sqrtd) / a;
+            root = (-half_b + sqrtd) / a;
             if root < t_min || root > t_max {
                 return None;
             }