Fix a memory leak

We create dirtyOnScreenWidgets even if QT_NO_OPENGL is defined,
but dtor deletes this object  only if no QT_NO_OPENGL is defined.
Also, copy ctor/assignment operator must be disabled for such a class.

Task-number: QTBUG-47499
Change-Id: I9f8b5721ff6324fa7b28b715af68d0a5496c5eb5
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/widgets/kernel/qwidgetbackingstore.cpp

@@ -763,9 +763,8 @@ QWidgetBackingStore::~QWidgetBackingStore()
 
 #ifndef QT_NO_OPENGL
     delete widgetTextures;
-    delete dirtyOnScreenWidgets;
 #endif
-    dirtyOnScreenWidgets = 0;
+    delete dirtyOnScreenWidgets;
 }
 
 //parent's coordinates; move whole rect; update parent and widget

src/widgets/kernel/qwidgetbackingstore_p.h

@@ -301,6 +301,8 @@ private:
     friend class QWidgetPrivate;
     friend class QWidget;
     friend class QBackingStore;
+
+    Q_DISABLE_COPY(QWidgetBackingStore)
 };
 
 QT_END_NAMESPACE