QRegion: explain why there's an apparently unused variable

The variable is there to work around an aliasing issue.
Since this code is likely coming straight from Qt 2 or 3,
I'm not doing any major modification, just explaining why that
variable is necessary and mark it as unused (so that a static
analyzer that knows about value classes won't complain that
we're creating a QVector without using it).

Change-Id: Ie8777563724ec3c0bf97d8bc24e1b184fe26bd2f
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Reviewed-by: Sérgio Martins <sergio.martins@kdab.com>

src/gui/painting/qregion.cpp

@@ -2281,7 +2281,14 @@ static void miRegionOp(QRegionPrivate &dest,
 
     dest.vectorize();
 
-    QVector<QRect> oldRects = dest.rects;
+    /*
+     * The following calls are going to detach dest.rects. Since dest might be
+     * aliasing *reg1 and/or *reg2, and we could have active iterators on
+     * reg1->rects and reg2->rects (if the regions have more than 1 rectangle),
+     * take a copy of dest.rects to keep those iteractors valid.
+     */
+    const QVector<QRect> destRectsCopy = dest.rects;
+    Q_UNUSED(destRectsCopy);
 
     dest.numRects = 0;