void Screensaver::update(double time)
{
    rotation = time*0.1;
    
    double timestep = (time-last_update);
    bool need_sort = false;
    
    std::vector<NumberStream>::iterator it;
    for (it = numbers.begin(); it != numbers.end(); ++it) {
        it->translate(0, timestep*it->get_speed(), 0);
        
        if(it->get_position().y > 8) {
            it->set_position(drand(-5, 5), -20, drand(-9, 1));
            it->set_speed(drand(0.1, 0.2));
            need_sort = true;
        }
    }
    if (need_sort) {
        std::sort(numbers.begin(), numbers.end(), DepthSortingComp());
    }

    last_update = time;
}