drawBackground();這個函數為什么是在gameloop里調用，gameloop()具體是怎么循環的
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();
}
2016-02-16
window.requestAnimFrame(gameloop)
這個方法使gameloop循環，具體看插件