Move bullets.

src/entity/bullet.cxx

@@ -16,7 +16,7 @@ using namespace danmaku;
 bullet::bullet(int damage, math::vec2 const & vel, graphics::object const & obj)
     : engine::collidable(obj), damage_(damage), velocity_(vel) {}
 
-void bullet::update(float delta) {}
+void bullet::update(float delta) { move(velocity_ * delta); }
 
 void bullet::set_position(math::vec2 const & ll, math::radian facing) {
   auto & size = render_info_.location.size;