UI: Fix screen resolution for canvas size

In #3988, @RytoEx mentioned that the recent Qt upgrade to 5.15.2
introduced a regression in which Qt begins returning DPI/scaling
aware resolutions for each screen. While this was fixed for
new profiles, this was not reflected in the choice for the
Canvas (Base) Resolution in the Settings screen yet.

This commits fixes this issue, and calculates the correct
physical screen size again, respecting per-screen DPI scaling.