todo

有关mpeg2的deocde代码

* we don't have any functions for reading / printing TXT_MGI
* rename zero_1 and add bitfields / macro constants in pgcit_srpt
  - done, are any of the other pgci_t pgci_ut_t pgci_lu_t affected?
* if an encrypted block is being read and libdvdcss is unavailable,
  we should probably return 0 to help it fail.