Report font tables are R/O blobs

There is no point in modifying font tables, so mark them RO for clarity.

Change-Id: I8ddbda9ace9c5a0eeabc0c5e5f64558060b5ee69
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/gui/text/qharfbuzzng.cpp

@@ -590,7 +590,7 @@ _hb_qt_reference_table(hb_face_t * /*face*/, hb_tag_t tag, void *user_data)
         length = 0;
 
     return hb_blob_create(const_cast<const char *>(buffer), length,
-                          HB_MEMORY_MODE_WRITABLE,
+                          HB_MEMORY_MODE_READONLY,
                           buffer, free);
 }