元素页面位置[left,top]获取

var getCanvasPos = function(canvas) {
        var left = 0;
        var top = 0;
        
        // 元素在页面的位置left和top
        while (canvas.offsetParent) {
            left += canvas.offsetLeft;
            top += canvas.offsetTop;
            canvas = canvas.offsetParent;
        }
        return [left, top];
    }