[Reg 5.9.0->5.9.2] Setting anti-aliasing has no effect when rendering to a frame buffer object

Anti-aliasing can be set to QPainter with http://doc.qt.io/qt-5/qpainter.html#setRenderHint or to QQuickPaintedItem with http://doc.qt.io/qt-5/qquickpainteditem.html#setAntialiasing.

In a test application both seem to work with the default setting when QPainter paints to QImage using the raster engine. But when frame buffer  object is used, setting anti-aliasing does not have an effect.

From http://doc.qt.io/qt-5/qquickpainteditem.html#RenderTarget-enum  QQuickPaintedItem::FramebufferObject: "Painting can be faster as no texture upload is required, but anti-aliasing quality is not as good as if using an image.

This regressed between 5.9.0 and 5.9.2

The behavior is different from 5.5.1