offscreen: Return the requested window format

Not the default constructed QSurfaceFormat.

If the window had alphaBufferSize set to some value, we want that
value returned from the platformwindow too.

Pick-to: 6.8
Task-number: QTBUG-136755
Change-Id: Ie0b51b773e0127cac797607a52947bba7ec5eba0
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit d22660a09d36818d4c39a6e6b85430c377849dfe)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/offscreen/qoffscreenwindow.cpp

@@ -130,6 +130,11 @@ WId QOffscreenWindow::winId() const
     return m_winId;
 }
 
+QSurfaceFormat QOffscreenWindow::format() const
+{
+    return window()->requestedFormat();
+}
+
 QMargins QOffscreenWindow::frameMargins() const
 {
     return m_margins;

src/plugins/platforms/offscreen/qoffscreenwindow.h

@@ -26,6 +26,7 @@ public:
     void requestActivateWindow() override;
 
     WId winId() const override;
+    QSurfaceFormat format() const override;
 
     static QOffscreenWindow *windowForWinId(WId id);