h264parse: handle DirecTV 708 closed captions
Parse DirecTV closed captions.
For difference between ATSC and DirecTV, see
https://en.wikipedia.org/wiki/CEA-708#Picture_User_Data
H.264/H.265 supplemental enhancement information prefix
Note: DirecTV does not appear in MPEG content, according to docs, so no need to also handle this in mpeg parser.
Edited by Aaron Boxer