message: Allow angle brackets in MTLX serialization
  This changelist modifies PugiXML to allow angle brackets in MTLX serialization, so that geometry tokens (e.g. <UDIM>) are written out unmodified.
diff: @@ -3922,11 +3922,13 @@ PUGI__NS_BEGIN
  ++s;
  break;
  case '<':
  - writer.write('&', 'l', 't', ';');
  + // MaterialX: Allow angle brackets in MTLX serialization.
  + writer.write(*s);
  ++s;
  break;
  case '>':
  - writer.write('&', 'g', 't', ';');
  + // MaterialX: Allow angle brackets in MTLX serialization.
  + writer.write(*s);
  ++s;
  break;
  case '"':
  