Fix image grab when XCB is rgb-swapping

The rgbSwap produces an image that fits the X-server, but not one that
fits our internal image definitions, so instead return our internal
image.

Task-number: QTBUG-56806
Change-Id: I25aedf7279bcd86792213b11dbd07a77b49538de
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

src/plugins/platforms/xcb/qxcbbackingstore.cpp

@@ -837,6 +837,9 @@ void QXcbBackingStore::endPaint()
 
 QImage QXcbBackingStore::toImage() const
 {
+    // If the backingstore is rgbSwapped, return the internal image type here.
+    if (!m_rgbImage.isNull())
+        return m_rgbImage;
     return m_image && m_image->image() ? *m_image->image() : QImage();
 }