Cannot run regression test on systems with high-dpi monitors

[ftilde]

When running on a system with a devicePixelRatio of for example 2, all screenshots will be double the expected size in each dimension. This is because the screenshot dimensions are equivalent to the physical canvas size, while the window size saved in the workspace corresponds to the logical canvas size.

I don't really know what to do here. I don't think we want to reduce the screenshot size on high-dpi screens by default. That would certainly be unexpected for the user (and I suspect that this might be quite hairy to implement).