Rename hb_buffer_clear() to hb_buffer_clear_contents()

The previous name was clashing with harfbuzz.old.  There are systems
that need to link both...

Clash-free now again.
diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index 48ec4a5..5386e36 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -193,7 +193,7 @@
 
 /* Like reset, but does NOT clear unicode_funcs. */
 void
-hb_buffer_clear (hb_buffer_t *buffer);
+hb_buffer_clear_contents (hb_buffer_t *buffer);
 
 /* Returns false if allocation failed */
 hb_bool_t