The WS_EX_LAYERED state of a window can be determined on creation, and
the logic to do so should be centralized to QWindowsWindow::setWindowLayered,
so that we don't have divergence.
This fixes an issue where child windows would only support transparency
if they had Qt::FramelessWindowHint set, as the QWindowBackingStore had
a different idea about when to enable WS_EX_LAYERED than QWindowsWindow.
Task-number: QTBUG-122590
Task-number: QTBUG-135859
Pick-to: 6.9 6.8
Change-Id: I453967a5a2ce8974cdd1dbf6e36327e97384c33d
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Zhao Yuhang <2546789017@qq.com>
Reviewed-by: Pavel Dubsky <pavel.dubsky@qt.io>
876bf141cf