xcb: Return standalone image from QXcbBackingStore::toImage()

Otherwise the original backingstore image will detach from the
m_xcb_image data on the next backingstore paint or scroll.

Pick-to: 6.2
Change-Id: I73f68d9c2e7c106951541831a5df8b97695f2001
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/plugins/platforms/xcb/qxcbbackingstore.cpp

@@ -850,7 +850,13 @@ QImage QXcbBackingStore::toImage() const
         return QImage();
 
     m_image->flushScrolledRegion(true);
-    return *m_image->image();
+
+    QImage image = *m_image->image();
+
+    // Return an image that does not share QImageData with the original image,
+    // even if they both point to the same data of the m_xcb_image, otherwise
+    // painting to m_qimage would detach it from the m_xcb_image data.
+    return QImage(image.constBits(), image.width(), image.height(), image.format());
 }
 
 QPlatformGraphicsBuffer *QXcbBackingStore::graphicsBuffer() const