When the devicePixelRatio changes, the Chart Cluster does not display correctly.

[seiyagg]

Browser: Chrome or Edge on Windows.
Step to reproduce:

1. Open example page: https://viglino.github.io/ol-ext/examples/map/map.clustering.html
2. Change devicePixelRatio to 200%
   
3. Refresh the page - the chart will be clear
   
4. Change devicePixelRatio to 100%
5. Zoom in/out by clicking the "+"/"-" button, then the chart cannot work well
   

I only used the example page to reproduce how to make the chart not display. I know that users won't frequently switch the pixelRatio in real usage, but in my SPA, if a user changes the pixelRatio on another page and then navigates back to the map, the chart won't display.