Windows and dialogs flash white before painting content on shown

When a Qt window is shown, it tends to show up with a white client area for a short time before the actual widgets can be painted on it. This can cause eye strain and other adverse effects, especially when a dark theme is used or when in low light environment.

This was reported to Krita (https://bugs.kde.org/show_bug.cgi?id=445968), Shotcut (https://github.com/mltframework/shotcut/issues/1168) and Sandboxie (https://github.com/sandboxie-plus/Sandboxie/issues/1231) by an end user. It can also be observed on the latest Qt Creator 6.0.0 (Built on Dec 1 2021 16:19:37), when opening Qt Creator, opening the Options and other dialogs. (Tested on Windows 10 build 19041, with a dark theme set in Qt Creator to make it more noticeable.)