[HarfBuzz] Better handling variation selectors

[Behdad Esfahbod]

Konstantin has brought up a few interesting issues regarding variation
selectors.  I'll summarize issues that I think can be improved:

  - Right now we don't handle a sequence of multiple variation selectors in a
row.  What *is* the best way to handle that?  Ideally the extra ones should
become visible.  Even a box is better than silently not showing them, right?

  - Now, Unicode says only registered variation sequences are valid.  I assume
we don't want to enforce that?

  - What should HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES do to variation
selectors?  Right now they do not become visible no matter what.  Is it
relevant whether the variation selector was actually used during glyph lookup
or not?

  - If the answer to the last question of the previous item is yes, then we
would new to change get_glyph() API to two separate get_glyph_variation() and
get_glyph() calls.  But that would be an API and ABI break :(.  Thinking about
it now, I wish I had done that from the beginning.

Thoughts?

-- 
behdad
http://behdad.org/