🐛 [BUG]设置图例的背景，只有第一个生效了

### 🐛 Bug description [Please make everyone to understand it]
想要给图例设置背景色，但是只第一个生效了

图例配置如下
`legend: {
position: 'right',
itemHeight: 30,
background: {
style: {
stroke: null,
fill: 'green',
}
}
}`

> 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.

基于这个示例： https://codesandbox.io/s/epic-sea-8yrxoj?file=/index.js

* _Required_ **Link to minimal reproduction**:

### 📷 Step to reproduce

### 🏞 Expected result

### 🚑 Any additional [like screenshots]

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

Start with the linked CodeSandbox reproduction, especially index.js, and inspect how the legend background configuration is applied when multiple legend items are rendered. Done means the configured green background appears for every legend item rather than only the first.

Written by the indexing model from the issue text.