Use reserve on QVector<Phase2Data>

Summary:
We know that the maximum size is the number of windows in stacking
order. It might be that some are skipped as they are not ready for
painting, but overall it's better to reserve a little bit too much
memory than to have a growing QVector.

Test Plan: Compiles

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18208

scene.cpp

@@ -207,6 +207,7 @@ void Scene::paintGenericScreen(int orig_mask, ScreenPaintData)
         paintBackground(infiniteRegion());
     }
     QVector<Phase2Data> phase2;
+    phase2.reserve(stacking_order.size());
     foreach (Window * w, stacking_order) { // bottom to top
         Toplevel* topw = w->window();
 
@@ -250,6 +251,7 @@ void Scene::paintSimpleScreen(int orig_mask, QRegion region)
     assert((orig_mask & (PAINT_SCREEN_TRANSFORMED
                          | PAINT_SCREEN_WITH_TRANSFORMED_WINDOWS)) == 0);
     QVector<Phase2Data> phase2data;
+    phase2data.reserve(stacking_order.size());
 
     QRegion dirtyArea = region;
     bool opaqueFullscreen(false);