layout: rework inline box area model

Specifying the background area as three LayoutUnit pairs sounds clever,
but breaks down for text-align: center.

* store line Y offset in LineBox - this is necessary to pass down areas
  we need to paint
* refactor addHorizontalLine loop; remove mystery "If necessary, update ictx's
  width" line that used to be labeled "TODO this seems meaningless" and indeed
  seems meaningless now (I am prepared to deeply regret this later)
* replace broken & convoluted startOffset/endOffset/size with a simple
  seq of offsets + sizes

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