fix corner bias in Vec3#random(random, true)

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

@@ -17,17 +17,29 @@ public record Vec3(double x, double y, double z) {
         assert Double.isFinite(x) && Double.isFinite(y) && Double.isFinite(z) : "x, y and z must be finite";
     }
 
+    /**
+     * {@return a uniformly random vector with components in the range [-1, 1)}
+     */
     public static @NotNull Vec3 random(@NotNull RandomGenerator random) {
-        return random(random, false);
+        return new Vec3(
-    }
-
-    public static @NotNull Vec3 random(@NotNull RandomGenerator random, boolean unit) {
-        var vec = new Vec3(
                 Math.fma(2, random.nextDouble(), -1),
                 Math.fma(2, random.nextDouble(), -1),
                 Math.fma(2, random.nextDouble(), -1)
         );
-        return unit ? vec.unit() : vec;
+    }
+
+    /**
+     * {@return a uniformly random unit vector}
+     */
+    public static @NotNull Vec3 random(@NotNull RandomGenerator random, boolean unit) {
+        if (!unit) return random(random);
+        Vec3 vec;
+        double squared;
+        do {
+            vec = random(random);
+            squared = vec.squared();
+        } while (squared > 1);
+        return vec.div(Math.sqrt(squared));
     }
 
     public static @NotNull Vec3 reflect(@NotNull Vec3 vec, @NotNull Vec3 normal) {