Add missing screen size conversions

[meehl]

GetScreenSize returns the size in physical units if the DPI_AWARE flag is passed to RenderInit. The error popup and restart screen have the wrong position and size at UI scaling factor != 1.0 due to missing conversions to virtual units.

This introduces a utility function for getting the virtual screen size and uses it to add the missing conversions.

Before screenshot:

After screenshot:

[meehl]

This also needs to be applied to PoB2. It looks like this can be automated by setting a label but i can also do it manually if wanted.

[Nightblade]

There's some GetScreenSize calls in src/Export/Launch.lua and src/Export/Main.lua, ~and don't forget to remove the stub from HeadlessWrapper.lua too~! 😁

edit: derp