main: Fix borderless fullscreen for high dpi scaled displays

On Windows, a borderless window will be treated the same as exclusive fullscreen
when the window geometry matches the physical dimensions of the screen.
However, with High DPI scaling, when the devicePixelRatioF() is > 1, the borderless
window apparently is not treated as exclusive fullscreen and functions correctly.
One can verify and replicate this behavior by using a high resolution (4K) display,
and switching between 100% and 200% scaling in Windows' display settings.
At 100%, without the addition of 1, it is treated as exclusive fullscreen.
At 200%, with or without the addition of 1, it is treated as borderless windowed.
Therefore, we can use (read: abuse) this difference in behavior to fix this issue for
those with higher resolution displays when the Qt scaling ratio is > 1.
Should this behavior be changed in the future, please revisit this workaround.