Qt/RenderWidget: Account for devicePixelRatio when auto-adjusting window size

[Techjar]

We want the game displayed at its actual resolution, not a scaled resolution. Fixes https://bugs.dolphin-emu.org/issues/11861

[BhaaLseN]

Do we handle moving the window between screens anywhere? If so, that place needs to be updated as well (assuming we declare PerMonitorDPI in our manifest)

[Techjar]

There doesn't appear to be anything handling movement between screens. Qt doesn't provide an elegant way to do it either (QWindow::screenChanged doesn't do what you think it does), so we'd have to handle the movement event and check if the screen ID changed ourselves.

[MayImilae]

Tested! So this fixes it on the primary display, 1:1 exactly as it should be! However, if I move the window to another display or even open dolphin on another display, the ui is always scaled based on the primary display. So my primary display is 200% scaling, but my side monitors are 150% scaling. On the primary monitor 2x native is correctly set to 1280 pixels wide, but on the secondary displays, it is incorrectly 960 pixels wide. No matter what I did, changing multiple resolutions to auto-adjust multiple times, moving the render window between displays, or opening Dolphin on those screens, it was always 960 pixels wide.

Well, it's still an improvement, but, hmm.

[spycrab]

See my comment above. Merging it as is would break building with CMake on Windows.

[Techjar]

Pinging @MayImilae to check the behavior of this PR again, since it's now using a different function to get the QScreen.

[Techjar]

We've verified that per-monitor DPI works now, so this should be good to merge! We noticed some other odd bugs in testing but they aren't in any way related to this PR, and also occur on master.

[spycrab]

Good to go now as far as I am concerned.

[jordan-woyak]

LGTM.