qml: let `grabToImage()` do the off-screen rendering in DragItem
Trying to use grabToImage() on an invisible item
reportedly caused issues, possibly due to lack of
polish call or children just not getting ready as
soon as they are made visible off-screen.
With Qt 6, this does not seem to apply anymore.
Instead of rendering off-screen manually, it should
be a better idea to just let grabToImage() do that
itself.
Request review @chub.