We were only dumping a PaintableBox' dimensions if its layout node was a
Layout::Box as well, causing us to not dump the dimensions of paintables
for inline nodes in the paintable tree.
We assume elsewhere that any abspos element's containing block must be
some kind of Layout::Box, so let's enforce that when deciding if a box
can be such a container.
This fixes a bad downcast on https://serpapi.com/