shape text 计算文字尺寸的过程中丢失 context 导致 measureText 时尝试新建 canvas

- [x] I have searched the [issues](https://github.com/antvis/f2/issues) of this repository and believe that this is not a duplicate.

### Reproduction link
[![Edit on CodeSandbox](https://codesandbox.io/static/img/play-codesandbox.svg)](https://codesandbox.io/s/cocky-hill-q41vu)

### Steps to reproduce

```js
chart.tooltip({
  showXTip: true
});
```

使用 `showXTip` 属性, 会调用 `graphic/shape/text.js` 中的 `calculateBox` 方法.

https://github.com/antvis/F2/blob/d9611fe40d9d80097137a28a09702c00ace21b36/src/graphic/shape/text.js#L147-L151

其中用到了 `util/dom.js` 中的 `measureText` 函数.
但不知是什么原因, 这中间的 context 丢失了. 导致 `measureText` 函数尝试创建新的 canvas 用来计算文字尺寸.

https://github.com/antvis/F2/blob/d9611fe40d9d80097137a28a09702c00ace21b36/src/util/dom.js#L148-L150

| Environment | Info |
|---|---|
| f2 | 3.7.0 |
| System | - |
| Browser | - |

	calculateBox() {
	const self = this;
	const attrs = self._attrs.attrs;
	const { x, y, textAlign, textBaseline } = attrs;
	let width = self._getTextWidth(); // attrs.width

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shape text 计算文字尺寸的过程中丢失 context 导致 measureText 时尝试新建 canvas #1015

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	if (!ctx) {
	ctx = document.createElement('canvas').getContext('2d');
	}