DPI scaling

[BruegelN]

This PR addresses #124

I've only tested this on my linux machine so far. So I'm happy to get some feedback for these changes.

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Here some screenshots from before these changes.
On a 4k screen with 100% scaling:

and with with 200% scaling:

The only noticeable difference is the size of the top bar.

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As of 40f4f27 on a 4k screen with 100% scaling:

and with 200% scaling:

[nmwsharp]

This is amazing, thanks so much, it's been on my TODO list for ages. We should definitely get it merged.

I'll want to test on some cross-platform machines to make sure we're getting that right (IIRC macOS in particular handles DPI differently). We should also think about trying to set this value reasonably by default based on the info we get from glfw. And add a UI slider! I briefly scrolled the code here now and it looks reasonable.