svg textPath中文字居中

参考资料：[How to Center Text inside an SVG Path](https://stackoverflow.com/questions/44860320/how-to-center-text-inside-an-svg-path)

```html
<text style="font-size: 24px;">
    <textPath xlink:href="#p1" startOffset="50%" text-anchor="middle">1test text</textPath>
</text>
```
关键是` startOffset="50%" text-anchor="middle"`