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 patch just adds the new root paintable and updates the tests
expectations. The next patch will move painting logic from the layout
viewport to the paint viewport.
Make sure the insets and margins calculated according to the spec are
not later ignored and ad-hoc recomputed in
layout_absolutely_positioned_element.
Use the static position calculation in a couple of places where the
spec (and comment) was indicating it should be used.
Fixes#19362
