videoparsers: Improvements to SEI data handling

• videoparseutils: Minor improvements to _parse_bar
  Use the enum type for the argument and loudly complain when not passing a place to output.

• videoparseutils: Minor improvements to _parse_afd

  • Add the missing field parameter and put the output parameter at the end.
  • Use a switch to verify valid values instead of hard-to-follow range checks.
  • Don't consider bad values a programming error, just a regular failure.
  • Set all data fields at the end so we can pass a pointer to an uninitialized structure without GCC complaining.

• videoparsers: Clear user data explicitly in _reset_frame
  This fits better with the model used by the parser elements. It also properly resets the data when the parser is reset.

• videoparsers: Clear user_data_unregistered explicitly in _reset_frame
  This fits better with the model used by the parser elements. It also properly resets the data when the parser is reset.

• videoparseutils: Store multiple user data unregistered messages
  A frame can have multiple unregistered messages attached. We need to store them all.

  For: #3008 (closed)

• videoparseutils: Don't double-attach AFD, Bar or unregistered data
  We already did this for captions. We also need to do it for the other meta types to ensure our parsers are idempotent and don't attach duplicates.