🐛 [BUG]散点图的散点如果在边界, 则很难触发tooltip

### 🐛 Bug description [Please make everyone to understand it]

> Please provide a link by forking these links [G2Plot](https://codesandbox.io/s/g2plot-issue-template-gzfdq?file=/index.ts) or GitHub repo, a minimal reproduction.

* _Required_ **Link to minimal reproduction**:
https://codesandbox.io/s/1jr3ep?file=/index.ts
### 📷 Step to reproduce
当有一个散点在图表边界时, 超出边界部分的元素无法触发tooltip


### 🏞 Expected result

### 🚑 Any additional [like screenshots]

* **G2Plot Version**:
* **Platform**:

Start with the linked CodeSandbox minimal reproduction at /index.ts and reproduce the scatter plot tooltip behavior when a point lies on the chart boundary. Trace the G2Plot scatter and tooltip entry points from that example; done means the portion of a boundary point extending beyond the plot area can still trigger its tooltip.

Written by the indexing model from the issue text.