optimize hitSphere()

in_one_weekend/src/vec3.zig

@@ -41,6 +41,10 @@ pub const Vec3 = packed struct {
         return math.sqrt(self.x * self.x + self.y * self.y + self.z * self.z);
     }
 
+    pub fn length_squared(self: Vec3) f32 {
+        return self.x * self.x + self.y * self.y + self.z * self.z;
+    }
+
     pub fn dot(u: Vec3, v: Vec3) f32 {
         return u.x * v.x + u.y * v.y + u.z * v.z;
     }