Pango Fc Shaper gives incorrect geometry information

[Simon Cozens <simon simon-cozens org>]

I've had some bug reports about my SILE typesetter providing badly 
letter-spaced output on Linux, when the output looks fine in OS X. I've 
tracked this down to pango_shape returning geometries which are being 
(erroneously) quantized to integer multiples of PANGO_UNITS.

For instance, the code below on OS X (using the pango-basic-coretext module) 
returns the correct answer, 7.070312. On Linux, it reports the width of a 12pt 
Gentium "z" as 7.000000pts. The width of an "m" is 12.851562pt on OS X, but 
12.000000pts on Linux.

I'm not sure what to do (other than shift to harfbuzz for shaping...) to get 
the correct results. Is it something inside Pango quantizing the geometries, 
or something inside FreeType?

#include <pango/pangocairo.h>

int main() {
  PangoFontMap * fm = pango_cairo_font_map_get_default ();
  PangoContext * pango = pango_font_map_create_context(fm);
  PangoAttrList * pal = pango_attr_list_new();
  PangoAttribute * pa1 = pango_attr_family_new("Gentium");
  PangoAttribute * pa2 = pango_attr_size_new(PANGO_SCALE * 12);
  PangoItem* charitem;
  PangoGlyphString* pgs = pango_glyph_string_new();
  PangoGlyphInfo g;

  pango_attr_list_insert(pal, pa1);
  pango_attr_list_insert(pal, pa2);
  charitem = pango_itemize(pango, "m", 0, 1, pal, NULL)->data;
  pango_shape("m", 1, &(charitem->analysis), pgs);
  g = (pgs->glyphs)[0];
  printf("%f\n",g.geometry.width/(double)PANGO_SCALE);
}