Use QVector in paintGenericScreen

Summary:
Given that QList<T> will allocate items on the heap when sizeof(T) is
bigger than sizeof(void *), which is the case for Phase2Data, use
QVector instead.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18198

scene.cpp

@@ -206,7 +206,7 @@ void Scene::paintGenericScreen(int orig_mask, ScreenPaintData)
     if (!(orig_mask & PAINT_SCREEN_BACKGROUND_FIRST)) {
         paintBackground(infiniteRegion());
     }
-    QList< Phase2Data > phase2;
+    QVector<Phase2Data> phase2;
     foreach (Window * w, stacking_order) { // bottom to top
         Toplevel* topw = w->window();