add a check to Vec3#unit to avoid unnecessary sqrt and division

src/main/java/eu/jonahbauer/raytracing/math/Vec3.java

@@ -270,7 +270,9 @@ public record Vec3(double x, double y, double z) {
      * {@return a unit vector with the same direction as this vector}
      */
     public @NotNull Vec3 unit() {
-        return div(length());
+        var squared = squared();
+        if (squared == 1) return this;
+        return div(Math.sqrt(squared));
     }
 
     /**