Feature request: Get device pixel ratio

[Nick-Mazuk]

It would be great to retrieve the device pixel ratio. Without it, you can't effectively use this crate for snapshot testing.

On Macs, the default device pixel ratio is 2.0. However, it seems like on GitHub Actions the device pixel ratio is 1.0. This means the dimensions of screenshots for local development and CI are different.

https://developer.mozilla.org/en-US/docs/Web/API/Window/devicePixelRatio