[HarfBuzz] Clusters chapter

[Khaled Hosny]

On Fri, Nov 02, 2018 at 03:47:31PM -0500, Nathan Willis wrote:
> Finally, I am adding a short "why your software cares about clusters"
> paragraph to the beginning. I've got cursor positioning, coloring
> diacritics, and line breaking in mind; anything else worth mentioning?

In addition to what Behdad mentioned, applying text attributes in
general (color, underline, overline, etc.), doing them properly requires
shaping first then finding which glyphs have which attributes using
cluster values. Justification can require character properties as well
(Japanese, Kashida, etc) but needs to be done after shaping, so mapping
glyphs back to input characters is needed.