QmlDesigner: Enable clipping in FormEditorItem

When resizing an item the instanceIsRenderPixmap can be bigger than
the bounding rect. In this case we get painting artefacts.

Change-Id: I6da43f16a1e85d045a0ebbb01db594979ad89622
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

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

@@ -338,6 +338,9 @@ void FormEditorItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *,
     if (isInStackedContainer)
         showPlaceHolder = qmlItemNode().instanceIsRenderPixmapNull() && isContentVisible();
 
+    painter->setClipRegion(m_boundingRect.toRect());
+    painter->setClipping(true);
+
     if (!hideCompletely) {
         if (showPlaceHolder) {
             if (scene()->showBoundingRects() && m_boundingRect.width() > 15 && m_boundingRect.height() > 15)