qml: consider device pixel ratio for round image source size

This was not a requirement before as it was done within the round image directly, but now that we use the default image QQuickImage, we need this like we do for normal images.

I do not want to do this within RoundImage.qml directly, for the same reason QQuickImage does not do that itself.

I also do not think having ScaledRoundImage.qml is a good idea.

Request review @chub.