When determining the content/margin box rects within their ancestor's
coordinate space, we were returning early if the passed in values
already belonged to the requested ancestor. Unfortunately, we had
already applied the used values' offset to the rect, which is the offset
to the ancestor's ancestor.
This simplifies the logic to always apply the rect offset after checking
if we've reached the ancestor. Fixes determining float intrusions inside
block elements with `margin: auto` set.
Fixes#4083.
