[HarfBuzz] Control characters inside ligatures

[Khaled Hosny]

Hi,

I just noticed that when there is a control character between character
that form a ligature, there is a zero width space after the ligature
with a cluster value of the first character in the ligature, for
example:

$ hb-unicode-encode U+0066,U+200C,U+0069 | hb-shape amiri-regular.ttf
[f_i=0+1064|space=0+0]

or 

$ hb-unicode-encode U+0066,U+00AD,U+0069 | hb-shape amiri-regular.ttf 
[f_i=0+1064|space=0+0]

This is rather surprising as I was expecting the control character to be
consumed inside the ligature and only the ligature glyph would remain. I
think the current behaviour makes mapping glyphs to text indices harder
in this case. WDYT?

Regards,
Khaled