[util] In --debug mode, duplicate font data

This has the effect that the font data will end up in a memory
section malloc()ed exactly to its size.  This gives us better
valgrind detection of out-of-bounds access.

Previously, the font data was placed in a mmap()ed section or
GString-allocated area, which didn't have proper protections
at the end when running under valgrind.

util/options.cc

@@ -538,6 +538,9 @@ font_options_t::get_font (void) const
       }
     }
 
+    if (debug)
+      mm = HB_MEMORY_MODE_DUPLICATE;
+
     blob = hb_blob_create (font_data, len, mm, user_data, destroy);
   }