Item.grabToImage() has AA edges assuming the background is white

You can see this with src/quick/doc/snippets/qml/externaldrag.qml : drag the green rectangle, it uses grabToImage() to create a DnD pixmap, and you can see a white halo around the rectangle's radius as you are dragging, although the far corners are transparent. Setting antialiasing: false is a workaround. Seems like it should just have alpha falling off around the edges instead of fading to white.