This is poorly factored. TextNode needs to know whether the most
recently inserted fragment on the current line was empty or ended in
whitespace. This is used when deciding what to do with leading
whitespace in text nodes.
Let's keep this working for now, but we should eventually sort this out
and make text chunk iteration not depend on this information.
When collapsing whitespace, we can skip over all-whitespace chunks at
the start of each line, and immediately following fragments that
themselves end in whitespace.
This patch adds a new mechanism that allows InlineFormattingContext to
build line boxes incrementally instead of all-in-one go.
Incremental build will eventually allow much better support for CSS
floating objects.
