Support scripts that are written both LTR and RTL

Right now only Old Italic is marked as such.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1000

test/api/test-common.c

diff --git a/test/api/test-common.c b/test/api/test-common.c
index 74b50be..f6f0d48 100644
--- a/test/api/test-common.c
+++ b/test/api/test-common.c
@@ -173,6 +173,7 @@
 
   g_assert_cmpint (hb_script_get_horizontal_direction (HB_SCRIPT_LATIN), ==, HB_DIRECTION_LTR);
   g_assert_cmpint (hb_script_get_horizontal_direction (HB_SCRIPT_ARABIC), ==, HB_DIRECTION_RTL);
+  g_assert_cmpint (hb_script_get_horizontal_direction (HB_SCRIPT_OLD_ITALIC), ==, HB_DIRECTION_INVALID);
   g_assert_cmpint (hb_script_get_horizontal_direction (hb_script_from_iso15924_tag (wWyZ)), ==, HB_DIRECTION_LTR);
 }