[font] Load named-instance if face index top bits are set

This matches FreeType behavior.

Fixes https://github.com/harfbuzz/harfbuzz/issues/3348

src/hb-face.cc

@@ -205,6 +205,10 @@ _hb_face_for_data_reference_table (hb_face_t *face HB_UNUSED, hb_tag_t tag, void
  * is ignored.  Otherwise, only the lower 16-bits of @index are used.
  * The unmodified @index can be accessed via hb_face_get_index().</note>
  *
+ * <note>Note: The high 16-bits of @index, if non-zero, are used by
+ * hb_font_create() to load named-instances in variable fonts.  See
+ * hb_font_create() for details.</note>
+ *
  * Return value: (transfer full): The new face object
  *
  * Since: 0.9.2

src/hb-font.cc

@@ -1521,6 +1521,13 @@ _hb_font_create (hb_face_t *face)
  *
  * Constructs a new font object from the specified face.
  *
+ * <note>Note: If @face's index value (as passed to hb_face_create()
+ * has non-zero top 16-bits, those bits minus one are passed to
+ * hb_font_set_var_named_instance(), effectively loading a named-instance
+ * of a variable font, instead of the default-instance.  This allows
+ * specifying which named-instance to load by default when creating the
+ * face.</note>
+ *
  * Return value: (transfer full): The new font object
  *
  * Since: 0.9.2
@@ -1535,6 +1542,9 @@ hb_font_create (hb_face_t *face)
   hb_ot_font_set_funcs (font);
 #endif
 
+  if (face && face->index >> 16)
+    hb_font_set_var_named_instance (font, (face->index >> 16) - 1);
+
   return font;
 }