Implement Unicode compatibility decompositions

Based on patch from Philip Withnall.
https://bugs.freedesktop.org/show_bug.cgi?id=41095
diff --git a/src/hb-unicode-private.hh b/src/hb-unicode-private.hh
index 1ce5adc..ba791eb 100644
--- a/src/hb-unicode-private.hh
+++ b/src/hb-unicode-private.hh
@@ -50,6 +50,7 @@
   HB_UNICODE_FUNC_IMPLEMENT (script) \
   HB_UNICODE_FUNC_IMPLEMENT (compose) \
   HB_UNICODE_FUNC_IMPLEMENT (decompose) \
+  HB_UNICODE_FUNC_IMPLEMENT (decompose_compatibility) \
   /* ^--- Add new callbacks here */
 
 /* Simple callbacks are those taking a hb_codepoint_t and returning a hb_codepoint_t */