Map math script in old_tag_from_script

Fixes #162

Author: dfrg

src/hb/common.rs

@@ -469,7 +469,7 @@ pub mod script {
     pub const TODHRI: Script = Script::from_bytes(b"Todr");
     pub const TULU_TIGALARI: Script = Script::from_bytes(b"Tutg");
 
-    pub const SCRIPT_MATH: Script = Script::from_bytes(b"Zmth");
+    pub const MATH: Script = Script::from_bytes(b"Zmth");
 
     // https://github.com/harfbuzz/harfbuzz/issues/1162
     pub const MYANMAR_ZAWGYI: Script = Script::from_bytes(b"Qaag");

src/hb/tag.rs

@@ -237,6 +237,8 @@ fn new_tag_from_script(script: Script) -> Option<hb_tag_t> {
 fn old_tag_from_script(script: Script) -> hb_tag_t {
     // This seems to be accurate as of end of 2012.
     match script {
+        script::MATH => hb_tag_t::from_bytes(b"math"),
+
         // Katakana and Hiragana both map to 'kana'.
         script::HIRAGANA => hb_tag_t::from_bytes(b"kana"),