Make WindowQuadList a QVector

In my last profiling session I did see a lot of WindowQuadList creation
cost. We know QList is a bit more expensive, so let's just use QVector.

libkwineffects/kwineffects.h

@@ -2574,7 +2574,7 @@ private:
 };
 
 class KWINEFFECTS_EXPORT WindowQuadList
-    : public QList< WindowQuad >
+    : public QVector<WindowQuad>
 {
 public:
     WindowQuadList splitAtX(double x) const;