src/ispriter.js
function setPxValue(style, attr, newValue) {
    var value;
    if (style[attr]) {
        value = parseInt(style[attr]);
    } else {
        value = 0;
        style[style.length++] = attr;
    }
    value = value - newValue;
    value = value ? value + 'px' : '0';
    style[attr] = value;
}
function setPxValue(style, attr, newValue) {
    var value;
    if (style[attr]) {
        value = parseInt(style[attr]);
    } else {
        value = 0;
        style[style.length++] = attr;
    }
    value = value - newValue;
    value = value ? value + 'px' : '0';
    style[attr] = value;
}
调整 样式规则的像素值, 如果原来就有值, 则在原来的基础上变更