QWindowsVistaStyle: Fill menu popup with theme color

The background for menu items is not drawn and can lead to a transparent
background when switching from QWindows11Style to QWindowVistaStyle at
runtime. This patch fills the background of menu items with
MENU_POPUPBACKGROUND.

Fixes: QTBUG-125474
Change-Id: I246db15ec821f9bc46ba410293c41b7e24548716
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
(cherry picked from commit 3de6d6f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/styles/modernwindows/qwindowsvistastyle.cpp

@@ -2836,6 +2836,11 @@ void QWindowsVistaStyle::drawControl(ControlElement element, const QStyleOption
             }
             QRect rect = option->rect;
 
+            //fill popup background
+            QWindowsThemeData popupbackgroundTheme(widget, painter, QWindowsVistaStylePrivate::MenuTheme,
+                             MENU_POPUPBACKGROUND, stateId, option->rect);
+            d->drawBackground(popupbackgroundTheme);
+
             //draw vertical menu line
             if (option->direction == Qt::LeftToRight)
                 checkcol += rect.x();