解决在android里面（三星s3）等一些4.1系统的手机上cancas刮刮乐无效

 function drawcircle(id,x,y,r){
                var ca = document.getElementById(id);
                var cans = ca.getContext("2d");
                cans.globalCompositeOperation = "destination-out";
                cans.beginPath();
                cans.arc(x,y,r,0,Math.PI*2);
                //解决兼容
                ca.style.display = ‘none’;
                ca.offsetHeight;
                ca.style.display = ‘inherit’; 
                //解决兼容
                cans.fillStyle="#000";
                cans.fill();
            }
这样可以解决

参考自：http://bbs.csdn.net/topics/390808206