Widgets: Use accelerated scroll when scrolled widget is overlapped

Accelerated scrolling in widgets is disabled when scrolled widget is overlapped by other widget(s). It it possible to remove this limitation.

Common use cases:

• faster scrolling when using "StyleHint::SH_ScrollBar_Transient",
• faster scrolling of zoomed image with semi-transparent thumbnail.

The attachment (QTBUG-64504.zip) contains testing software for QWidgetPrivate::moveRect() function. It can toggle overlay widgets, start a simple benchmark or toggle QOpenGLWidget. Performance can be tested together with commit from QTBUG-64414 on X11 platform, otherwise the difference will be negligible. Toggling QOpenGLWidget is useful for testing stability of QTBUG-64414.
The attachment (QTBUG-64504-scrollrect.zip) contains testing software for QWidgetPrivate::scrollRect() function. This software contains less functionality, but it can check if the second scrolling function also works properly.