HiDPI on Windows: Hairlines around corners

This is a workaround:

```
            path.addRect(0, 0, radius+0.3, radius+0.3)

            path.addRect(self.width()-radius-0.3, 0, radius+0.3, radius+0.3)
```

The proper solution would possibly involve `device_pixel_ratio = self.devicePixelRatioF()`?