Get region of overlapped widgets and scroll only non-overlapped parts
of image. Next, schedule an update for overlapped widgets region.
This patch improves scrolling performance when scrolled widget has
overlapped widgets.
Common use cases:
- faster scrolling when using "StyleHint::SH_ScrollBar_Transient",
- faster scrolling of zoomed image with semi-transparent thumbnail.
Accelerated scrolling with overlapped widgets is not available when
scale factor is non-integer.
Task-number: QTBUG-64504
Change-Id: I8337d3bc756e50f7d31cdc7979ccf86dc5c3695f
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
5b09346cf4