Offscreen: Implement QPlatformBackingStore::toImage

This makes testing of actually produced output easy in unit tests.

Pick-to: 6.3
Task-number: QTBUG-99962
Change-Id: Ia806539230af12d1eae1e31ef7a47155d9bc1bed
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/offscreen/qoffscreencommon.h

@@ -106,6 +106,7 @@ public:
     bool scroll(const QRegion &area, int dx, int dy) override;
 
     QPixmap grabWindow(WId window, const QRect &rect) const;
+    QImage toImage() const override { return m_image; }
 
     static QOffscreenBackingStore *backingStoreForWinId(WId id);