[HarfBuzz] hb_glyph_position_t::x_offset and scaled FreeType fonts

[Jamie Dale]

Hey all,

I'm running HarfBuzz 1.2.4 with a modified hb_font implementation based on
the FreeType implementation in the vanilla version.

One of the differences is that I pass through the FreeType scale as
user-data and apply it when getting the metrics from FreeType (via
FT_Set_Transform).

We've been running with this for a while, and haven't noticed any issues
until today when we tried it with some Thai text and found that the
hb_glyph_position_t::x_offset was coming back wrong once the text was
scaled (weirdly this same issue doesn't seem to affect
hb_glyph_position_t::y_offset).

When the scale is 1, everything is fine, but when the scale is <1 the glyph
is too far right (suggesting that the offset is too large), and when the
scale is >1 the glyph is too far left (suggesting that the offset is too
small).

That would suggest to me that the value I'm getting back hasn't been scaled
appropriately for the actual FreeType glyph. Unfortunately I'm not familiar
enough with how HarfBuzz calculates that value to know where that lack of
scale is happening.

I do notice that there's no FreeType override for the
set_glyph_h_origin_func from hb_font. Could that be the issue?

The Thai text being used is ฝึ (U+0E1D, U+0E36) and the font is Tahoma
(available in Windows).

Has anyone else attempted something like this?

Thanks,
Jamie.
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