HTML5 Canvas是否支持双缓冲？
要在画布上进行双缓冲，请创建第二个canvas元素并绘制到该元素。之后，使用drawImage()方法将图像绘制到第一个画布上，
// canvas element
var canvas1 = document.getElementById('canvas');
var context1 = canvas1.getContext('2d');
//缓冲画布
var canvas2 = document.createElement('canvas');
canvas2.width = 250;
canvas2.height =250;
var context2 = canvas2.getContext('2d');
//在画布上创建
context2.beginPath();
context2.moveTo(10,10);
context2.lineTo(10,30);
context2.stroke();
//渲染缓冲的画布
context1.drawImage(canvas2, 0, 0);