drawBackground();这个函数为什么是在gameloop里调用，gameloop()具体是怎么循环的
function gameloop(){
window.requestAnimFrame(gameloop); //setInterval,setTimeout,frame per second
//console.log("gameloop");
var now = Date.now();
deltaTime = now - lastTime;
lastTime = now;
//console.log(deltaTime);
drawBackground();
}
