Vistastyle paints invalid pixel data in backing store

As revealed by QTBUG-60562, the windows vista style paints invalid background pixels into the backing store. This appears to be because QWindowsXPStylePrivate::drawBackgroundDirectly (src\widgets\styles\qwindowsxpstyle.cpp:707) does not correct the alpha values like QWindowsXPStylePrivate::drawBackgroundThruNativeBuffer does. So we end up with a whtle background fill with alpha set to 0, which is invalid in premultiplied data.

Forcing canDrawDirectly to 'false' in QWindowsXPStylePrivate::drawBackground solves the issue by always using drawBackgroundThruNativeBuffer, but probably has a performance cost.