add alternate diffuse method

in_one_weekend/src/vec3.zig

@@ -87,6 +87,20 @@ pub fn random_in_unit_sphere(rng: *Random) Vec3 {
     }
 }
 
+pub fn random_unit_vector(rng: *Random) Vec3 {
+    return unitVector(random_in_unit_sphere(rng));
+}
+
+pub fn random_in_hemisphere(rng: *Random, normal: Vec3) Vec3 {
+    const in_unit_sphere = random_in_unit_sphere(rng);
+    if (in_unit_sphere.dot(normal) > 0) {
+        // In the same hemisphere as the normal
+        return in_unit_sphere;
+    } else {
+        return in_unit_sphere.mul(-1);
+    }
+}
+
 pub const Point3 = Vec3;
 
 const assert = @import("std").debug.assert;