Fix sbix glyph extents

* The ‘height’ needs to be negated since the API returns “distance from
  top to bottom side”.
* Similarly, the ‘y_offset‘ needs to be added to the height to get the
  ‘y_bearing’, since sbix’s offset is “the point in the glyph relative
  to its lower-left corner which corresponds to the origin” while
  ‘y_bearing’ is the “top side of glyph from origin”.

With these changes the sbix glyph metrics return values similar to other
tables, as they were otherwise unusable.

src/hb-ot-color-sbix-table.hh

@@ -235,9 +235,9 @@ struct sbix
       const PNGHeader &png = *blob->as<PNGHeader>();
 
       extents->x_bearing = x_offset;
-      extents->y_bearing = y_offset;
+      extents->y_bearing = png.IHDR.height + y_offset;
       extents->width     = png.IHDR.width;
-      extents->height    = png.IHDR.height;
+      extents->height    = -png.IHDR.height;
 
       /* Convert to font units. */
       if (strike_ppem)

test/api/test-ot-color.c

@@ -426,9 +426,9 @@ test_hb_ot_color_png (void)
   g_assert (strncmp (data + 1, "PNG", 3) == 0);
   hb_font_get_glyph_extents (sbix_font, 1, &extents);
   g_assert_cmpint (extents.x_bearing, ==, 0);
-  g_assert_cmpint (extents.y_bearing, ==, 0);
+  g_assert_cmpint (extents.y_bearing, ==, 800);
   g_assert_cmpint (extents.width, ==, 800);
-  g_assert_cmpint (extents.height, ==, 800);
+  g_assert_cmpint (extents.height, ==, -800);
   hb_blob_destroy (blob);
   hb_font_destroy (sbix_font);
 

test/shaping/data/in-house/tests/color-fonts.tests

@@ -1 +1,2 @@
 ../fonts/ee39587d13b2afa5499cc79e45780aa79293bbd4.ttf:--font-funcs=ot --show-extents:U+1F42F:[gid1=0+2963<0,2179,2963,-2789>]
+../fonts/fcbaa518d3cce441ed37ae3b1fed6a19e9b54efd.ttf:--font-funcs=ot --show-extents:U+1F600:[gid4=0+2550<0,1898,2555,-2405>]