Separate GlyphID from HBUINT16

For stricter enforcement.

src/hb-open-type.hh

diff --git a/src/hb-open-type.hh b/src/hb-open-type.hh
index 7edd953..a47eb74 100644
--- a/src/hb-open-type.hh
+++ b/src/hb-open-type.hh
@@ -151,7 +151,7 @@
 };
 
 /* Glyph index number, same as uint16 (length = 16 bits) */
-typedef HBUINT16 GlyphID;
+struct GlyphID : HBUINT16 {};
 
 /* Script/language-system/feature index */
 struct Index : HBUINT16 {