Pass through unknown ISO 639-3 language tags to OpenType engine

In hb_ot_tag_from_language(), if first component of an unknown
language is three letters long, use it directly as OpenType language
tag (after case conversion and padding).

src/hb-ot-tag.cc

@@ -651,6 +651,14 @@ hb_ot_tag_from_language (hb_language_t language)
     return HB_TAG('Z','H','S',' ');
   }
 
+  s = strchr (lang_str, '-');
+  if (!s)
+    s = lang_str + strlen (lang_str);
+  if (s - lang_str == 3) {
+    /* Assume it's ISO-639-3 and upper-case and use it. */
+    return hb_tag_from_string (lang_str, s - lang_str) & ~0x20202000;
+  }
+
   return HB_OT_TAG_DEFAULT_LANGUAGE;
 }
 

test/test-ot-tag.c

@@ -213,12 +213,17 @@ test_ot_tag_language (void)
 
   test_tag_from_language ("dflt", "asdf-asdf-wer-x-hbot-zxc");
 
+  test_tag_from_language ("dflt", "xy");
+  test_tag_from_language ("XYZ", "xyz"); /* Unknown ISO 639-3 */
+  test_tag_from_language ("XYZ", "xyz-qw"); /* Unknown ISO 639-3 */
+
   /* Test that x-hbot overrides the base language */
   test_tag_from_language ("ABC", "fa-x-hbotabc-zxc");
   test_tag_from_language ("ABC", "fa-ir-x-hbotabc-zxc");
   test_tag_from_language ("ABC", "zh-x-hbotabc-zxc");
   test_tag_from_language ("ABC", "zh-cn-x-hbotabc-zxc");
   test_tag_from_language ("ABC", "zh-xy-x-hbotabc-zxc");
+  test_tag_from_language ("ABC", "xyz-xy-x-hbotabc-zxc");
 }
 
 int