Fix a crash of accessing deleted paintengine

When painting to QPixmap with QBlitterPaintEngine and dfb
PaintEngine have PaintDevice (pdev) as QImage. Painter code
uses PaintDevice::paintEngine() to refer to paintEngine.
This will became dangling pointer since QDirectFbBlitter::doLock()
will delete QImage. Instead return QPixmap as PaintDevice.

Change-Id: Idfac919b6438a82b412020e441e0a102e4a2a052
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>

src/gui/painting/qpaintengine_blitter.cpp

@@ -494,11 +494,12 @@ void QBlitterPaintEngine::clipEnabledChanged()
 
 bool QBlitterPaintEngine::begin(QPaintDevice *pdev)
 {
+    Q_D(QBlitterPaintEngine);
     bool ok = QRasterPaintEngine::begin(pdev);
 #ifdef QT_BLITTER_RASTEROVERLAY
-    Q_D(QBlitterPaintEngine);
     d->pmData->unmergeOverlay();
 #endif
+    d->pdev = pdev;
     return ok;
 }