Add --utf8-clusters

Also fix cairo cluster generation.

util/helper-cairo.hh

diff --git a/util/helper-cairo.hh b/util/helper-cairo.hh
index bc3fe1d..2f2c9d4 100644
--- a/util/helper-cairo.hh
+++ b/util/helper-cairo.hh
@@ -75,6 +75,7 @@
 			       hb_buffer_t         *buffer,
 			       const char          *text,
 			       unsigned int         text_len,
-			       double               scale);
+			       double               scale,
+			       hb_bool_t            utf8_clusters);
 
 #endif