Add hb_buffer_clear()

Which is like _reset(), but does NOT clear unicode-funcs.
diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index dc63d1b..fbd809c 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -135,6 +135,10 @@
 void
 hb_buffer_reset (hb_buffer_t *buffer);
 
+/* Like reset, but does NOT clear unicode_funcs. */
+void
+hb_buffer_clear (hb_buffer_t *buffer);
+
 /* Returns false if allocation failed */
 hb_bool_t
 hb_buffer_pre_allocate (hb_buffer_t  *buffer,