Support bottom-right border for iframe

When a playground is on a white background, the white background color of the iframe blends into the background, which looks bad.


There should be some way to set the bottom-right border of the iframe, to produce an effect like this:


Note you can't quite do this simply with a border around the entire editor because of inconsistencies in radius (maybe there is some other approach we could use to make this work better too... `mask-clip`?), and also because a full border doesn't always look as nice.