HighDPI: Repainting issues with HighDpiScaleFactorRoundingPolicy::PassThrough and FramelessWindowHint

This issue affects applications that wants to draw a custom frame instead of the standard system frame. The problem itself arises when the DPI scaling of the monitor is changed.

How to reproduce:

1. Open the `simplebrowser` (QWidget-based browser) example in Qt Creator
2. Set the High-DPI scaling policy in `main.cpp`: 

   QApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::PassThrough);
   

3. Add the `FramelessWindowHint` flag to `BrowserWindow`:

   setWindowFlag(Qt::FramelessWindowHint);
   

4. Start the application
5. Set the DPI scaling of the monitor to 100%
6. Set the DPI scaling of the monitor to 125%
7. Set the DPI scaling of the monitor to 100%

Expected result: the application scales properly, the widgets are re-drawn

Actual result: there are black areas where the widgets didn't repaint themselves, other areas are blurry or has other scaling artifacts.

What I did on the video (attachment):

1. Set the DPI scaling of the monitor from 100% to 125%
2. Set the DPI scaling of the monitor from 125% to 150%
3. Set the DPI scaling of the monitor from 150% to 100% (in most cases the issue comes when changing the scale factor from a higher value to a lower one)

As you can see, some widgets has been re-drawn by hovering on them. Also there are scaling glitches on the rendered website in the web view.

Additionally this problem doesn't only come with `FramelessWindowHint`. In one of our applications we don't use that flag. Instead, we handle `WM_NCCALCSIZE` and use `DwmExtendFrameIntoClientArea`. (This method also removes the frame, but leaves the shadow and works with Aero snap). The result is the same.

If I change the DPI scale policy to one of the rounding ones, all widgets are re-drawn properly, but these policies cause incorrect font size vs. widget size ratios (too small text and huge widgets, or too small widgets and big text).

Since Qt 6 doesn't have WebEngine, I've made a simple QWidget application for this test, incorporating a label and a button in a layout. This application produces the same problems.