Inline nodes in our layout tree have a position, so let's show it. By
centralizing the logic for this, block nodes now lose their redundant
'content-size' dump info which is already part of the box model dump.
This suits the spec a bit better, and exposes the fact that we were
allowing `::ImageButton` to use the button layout although it is never
specified that it should do so. Tests were rebaselined for this.
