add hittable spheres

in_one_weekend/src/world.zig

@@ -0,0 +1,21 @@
+const Sphere = @import("sphere.zig").Sphere;
+const HitRecord = @import("hittable.zig").HitRecord;
+const Ray = @import("ray.zig").Ray;
+
+pub const World = struct {
+    spheres: []const Sphere,
+
+    pub fn hit(self: World, ray: Ray, t_min: f32, t_max: f32) ?HitRecord {
+        var hitRecord: ?HitRecord = null;
+        var closest = t_max;
+
+        for (self.spheres) |sphere| {
+            if (sphere.hit(ray, t_min, closest)) |localHit| {
+                closest = localHit.t;
+                hitRecord = localHit;
+            }
+        }
+
+        return hitRecord;
+    }
+};