Although most dimensions include the `<legend>` of a `<fieldset>`, when
a background is defined for a `<fieldset>` we should clip it during
painting as to exclude the background of the `<legend>`.
We were not properly including the impact of the fieldset's legend in
the client height, which caused us to report values that were different
than those from other browsers.
PaintContext dates back to a time when display lists didn't exist and it
truly represented "paint context". Renaming it to better align with its
current role.
