QGraphicsEffect artifacts with partial updates

When widgets or graphicsitems receive partial updates (from exposures or cursor movement updates), rendering artifacts occur. Reproduced with opacity and blur effects. The first screenshot shows what happens if you add QGraphicsOpacityEffect to the textedit demo (just to illustrate), with default settings. The other two screenshots show QGraphicsBlurEffect applied to the text edit demo, and then to the demo embedded into a QGraphicsProxyWidget.

To reproduce these cases, you can modify the textedit demo by adding effects to the "mw" window. This diff shows how to embed the app into a proxy widget:

@@ -49,6 +50,12 @@ int main( int argc, char ** argv )
QApplication a( argc, argv );
TextEdit mw;
mw.resize( 700, 800 );

• mw.show();
  +
  + QGraphicsScene scene;
  + QGraphicsView view(&scene);
  + QGraphicsProxyWidget *prox = scene.addWidget(&mw, Qt::Window);
  + prox->setGraphicsEffect(new QGraphicsBlurEffect(prox));
  + view.show();
  +
  return a.exec();
  }