🤔 [QUESTION] 密度热力图数据变动时能否让背景图片始终显示

### 🐛 Question description [Please make everyone to understand it]
比如官网demo的例子中拖拽温度筛选时背景的图片会在数据变动时消失,能不能做到背景图片始终显示呢

### 💻 Link to minimal reproduction
[密度热力图](https://g2plot.antv.vision/zh/examples/heatmap/density#basic)

### 🏞 Expected result

### 🚑 Any additional [like screenshots]

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

Start with the linked density heatmap demo and reproduce the issue by dragging the temperature filter. Trace the demo's data-update path and verify that the background image remains visible while the heatmap data changes.

Written by the indexing model from the issue text.