// ----------------------------------------------- //
void keyboard(unsigned char key, int x, int y) {
    if (key == GLUT_KEY_ESCAPE)
        glutLeaveMainLoop();
    else if (key == GLUT_KEY_ENTER || key == ' ')
        scenes.next();
    else if (key == GLUT_KEY_BACKSPACE)
        scenes.prev();
    else scenes.current()->start();

}