QFbScreen: replace a use of an inefficent QList with QVector

A QPair<QRect,int> is larger than a void*, so better use
a vector to hold them.

Change-Id: I8f0e65836008803b7f619a22ab3a0db1bd493471
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/platformsupport/fbconvenience/qfbscreen_p.h

@@ -108,7 +108,7 @@ private:
     void generateRects();
 
     QPainter *mCompositePainter;
-    QList<QPair<QRect, int> > mCachedRects;
+    QVector<QPair<QRect, int> > mCachedRects;
     QList <QFbBackingStore*> mBackingStores;
 
     friend class QFbWindow;