pac_center_x = Window.GetWidth() / 2;
pac_center_y = Window.GetHeight() / 2;
pac_frame = 0;

fun refresh_callback() {
    pac_half_width = pacman_sprite.GetImage().GetWidth() / 2;
    pac_half_height = pacman_sprite.GetImage().GetHeight() / 2;

    pac_frame = (pac_frame + 1) % 4;
    pac_max_center_x = screen_width + pac_half_width;
    pac_center_x = (pac_center_x + whatever_step_you_wish) % pac_max_center_x;

    pacman_sprite.SetImage(pacman_images[pac_frame]);
    pacman_sprite.SetX(pac_center_x - pac_half_width);
    pacman_sprite.SetY(pac_center_y - pac_half_height);
    pacman_sprite.SetZ(5000);
}