[pango/synthetic-slant] Pass synthetic slant to harfbuzz

[Matthias Clasen <matthiasc src gnome org>]

commit 1a553ba64c43c13e37d54f1fb20a656ffb2ef067
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Sep 11 12:44:29 2021 -0400

    Pass synthetic slant to harfbuzz
    
    When our font matrix includes a slant, pass that information
    to HarfBuzz, so it can adjust its mark positioning, metrics
    like caret slope, etc.
    
    This commit corrects the code that pulls the font matrix out
    of fontconfig to properly take the flipped y axis between fontconfig
    and pango into account.
    
    Requires new api in HarfBuzz > 3.2

 pango/pangofc-font.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)
---
diff --git a/pango/pangofc-font.c b/pango/pangofc-font.c
index 03f15efa..9d555a37 100644
--- a/pango/pangofc-font.c
+++ b/pango/pangofc-font.c
@@ -964,10 +964,12 @@ pango_fc_font_create_hb_font (PangoFont *font)
   double x_scale, y_scale;
   double pixel_size;
   double point_size;
+  double slant G_GNUC_UNUSED;
 
   x_scale_inv = y_scale_inv = 1.0;
   pixel_size = 1.0;
   point_size = 1.0;
+  slant = 0.0;
 
   key = _pango_fc_font_get_font_key (fc_font);
   if (key)
@@ -988,11 +990,12 @@ pango_fc_font_create_hb_font (PangoFont *font)
         FcMatrixMultiply (&fc_matrix, &fc_matrix, fc_matrix_val);
 
       font_matrix.xx = fc_matrix.xx;
-      font_matrix.yx = fc_matrix.yx;
+      font_matrix.yx = - fc_matrix.yx;
       font_matrix.xy = fc_matrix.xy;
-      font_matrix.yy = fc_matrix.yy;
+      font_matrix.yy = - fc_matrix.yy;
 
       pango_matrix_get_font_scale_factors (&font_matrix, &x, &y);
+      slant = pango_matrix_get_slant_ratio (&font_matrix);
 
       x_scale_inv /= x;
       y_scale_inv /= y;
@@ -1003,6 +1006,7 @@ pango_fc_font_create_hb_font (PangoFont *font)
           x_scale_inv = -x_scale_inv;
           y_scale_inv = -y_scale_inv;
         }
+
       get_font_size (key, &pixel_size, &point_size);
     }
 
@@ -1017,6 +1021,10 @@ pango_fc_font_create_hb_font (PangoFont *font)
                      pixel_size * PANGO_SCALE * y_scale);
   hb_font_set_ptem (hb_font, point_size);
 
+#if HB_VERSION_ATLEAST (3, 3, 0)
+  hb_font_set_synthetic_slant (hb_font, slant);
+#endif
+
   if (key)
     {
       const FcPattern *pattern = pango_fc_font_key_get_pattern (key);