🐛 [BUG] 密度热力图 无法使用函数式设置color

### 🐛 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/silly-water-7on9vt?file=/index.ts
### 📷 Step to reproduce

### 🏞 Expected result

### 🚑 Any additional [like screenshots]

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

Start with the linked CodeSandbox minimal reproduction in index.ts and inspect how the density heatmap processes its color setting. Reproduce the failure, then verify that a functional color configuration works as expected in the same example.

Written by the indexing model from the issue text.