Roll HarfBuzz to 1.2.6

third_party/harfbuzz-ng/src/hb-ot-layout-gsub-table.hh

Index: third_party/harfbuzz-ng/src/hb-ot-layout-gsub-table.hh
diff --git a/third_party/harfbuzz-ng/src/hb-ot-layout-gsub-table.hh b/third_party/harfbuzz-ng/src/hb-ot-layout-gsub-table.hh
index 38c2c6405a32efd1f156d63488eebd2d6eb0c843..7de56cf2a2bbe6af77c6a9c7bc002d96f2dae738 100644
--- a/third_party/harfbuzz-ng/src/hb-ot-layout-gsub-table.hh
+++ b/third_party/harfbuzz-ng/src/hb-ot-layout-gsub-table.hh
@@ -1288,10 +1288,28 @@ GSUB::substitute_start (hb_font_t *font, hb_buffer_t *buffer)
 
   const GDEF &gdef = *hb_ot_layout_from_face (font->face)->gdef;
   unsigned int count = buffer->len;
+  hb_glyph_info_t *info = buffer->info;
   for (unsigned int i = 0; i < count; i++)
   {
-    _hb_glyph_info_set_glyph_props (&buffer->info[i], gdef.get_glyph_props (buffer->info[i].codepoint));
-    _hb_glyph_info_clear_lig_props (&buffer->info[i]);
+    unsigned int props = gdef.get_glyph_props (info[i].codepoint);
+    if (!props)
+    {
+      /* Never mark default-ignorables as marks.
+       * They won't get in the way of lookups anyway,
+       * but having them as mark will cause them to be skipped
+       * over if the lookup-flag says so, but at least for the
+       * Mongolian variation selectors, looks like Uniscribe
+       * marks them as non-mark.  Some Mongolian fonts without
+       * GDEF rely on this.  Another notable character that
+       * this applies to is COMBINING GRAPHEME JOINER. */
+      props = (_hb_glyph_info_get_general_category (&info[i]) !=
+	       HB_UNICODE_GENERAL_CATEGORY_NON_SPACING_MARK ||
+	       _hb_glyph_info_is_default_ignorable (&info[i])) ?
+	      HB_OT_LAYOUT_GLYPH_PROPS_BASE_GLYPH :
+	      HB_OT_LAYOUT_GLYPH_PROPS_MARK;
+    }
+    _hb_glyph_info_set_glyph_props (&info[i], props);
+    _hb_glyph_info_clear_lig_props (&info[i]);
     buffer->info[i].syllable() = 0;
   }
 }