[HarfBuzz] Zero-width joiner has width

[Simon Cozens]

On 09/08/2015 09:15, Simon Cozens wrote:
> OK, I'm now trying to set the scaling to the font upem, but I'm not
> getting that working either.

After a bit of fiddling I have now got it working, using:
    hb_face_t* hbFace = hb_ft_face_create_cached(face);
    hb_ft_font = hb_font_create (hbFace);
    unsigned int upem = hb_face_get_upem(hbFace);
    hb_font_set_scale(hb_ft_font, upem, upem);
    hb_ft_font_set_funcs(hb_ft_font);

It's about 50% slower than using Freetype functions directly, which is a
big shame(!), but computers always get faster and I'm sure this way
gives me more flexibility later...

See latest version:
https://github.com/simoncozens/sile/blob/master/src/justenoughharfbuzz.c#L263