The rationale here is that every cell in an area needs to have space
reserved around it, requests have to be fully margin inclusive...
cells need to have the full size fed as the "background area" and
the "cell area" has margins removed... This will be used by GtkTreeViewColumn
to set the focus line width so that cells can paint a background on the
full background, then render themselves into the cell area... and parents
can go ahead and draw focus and other indicators on the background area
but outside of the cell area.