[HarfBuzz] Mark zeroing for East Asian scripts

[Jonathan Kew]

On 26/1/15 22:53, Behdad Esfahbod wrote:
> Jonathan,
>
> Trying with sequence of U+308F,3099,308F with NotoSansJP, looks like Uniscribe
> doesn't zero the mark advance but we do.  The font has bad data for this mark,
> but I want to fix the logic discrepancy.
>
> So I'll probably add a new (null-ish) shaper for HIRAGANA, KATAKANA, and HAN,
> and use HB_OT_SHAPE_ZERO_WIDTH_MARKS_NONE.
>
> WDYT?  What to call that shaper?

Seems reasonable to me. In practice, it looks like when fonts support 
the combining versions of the sound marks at all (some don't include 
them), they do it by ligating them with the preceding kana character, 
rather than just positioning a mark on the standard glyph (which would 
often clash, or at least look excessively crowded).

As such, zeroing the "mark" advance for a font that didn't do the 
ligation is not likely to be useful. So if Uniscribe doesn't do it (and 
neither does Cocoa/Core Text, afaict), let's just follow their lead.

Call it hb_ot_complex_shaper_eastasian? hb_ot_complex_shaper_han_kana?

JK