Update to Unicode 12.0.0
diff --git a/src/gen-vowel-constraints.py b/src/gen-vowel-constraints.py
index b7f6be2..e4dc95d 100755
--- a/src/gen-vowel-constraints.py
+++ b/src/gen-vowel-constraints.py
@@ -212,6 +212,7 @@
 print ('  {')
 print ('    if (buffer->idx < count)')
 print ('      buffer->next_glyph ();')
+print ('    buffer->swap_buffers ();')
 print ('  }')
 print ('}')