QmlDesigner.FormEditor: Fix painting of the background

Caching of the QGraphicsView is broken.

Task-number: QTCREATORBUG-5258
Change-Id: I7db2bff4809b592aaccfa74c37a3212edb29d403
Reviewed-on: http://codereview.qt.nokia.com/1239


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Marco Bubke <marco.bubke@nokia.com>

src/plugins/qmldesigner/components/formeditor/formeditorgraphicsview.cpp

@@ -46,8 +46,7 @@ FormEditorGraphicsView::FormEditorGraphicsView(QWidget *parent) :
     setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
     setResizeAnchor(QGraphicsView::AnchorViewCenter);
     setAlignment(Qt::AlignCenter);
-    setCacheMode(QGraphicsView::CacheBackground);
-//    setCacheMode(QGraphicsView::CacheBackground);
+    setCacheMode(QGraphicsView::CacheNone);
     setViewportUpdateMode(QGraphicsView::MinimalViewportUpdate);
     setOptimizationFlags(QGraphicsView::DontSavePainterState);
 //    setViewportUpdateMode(QGraphicsView::NoViewportUpdate);