Merge commit '50a1c66cf6ab7eb683daaa9e2da3869fa3a54609'

      * commit '50a1c66cf6ab7eb683daaa9e2da3869fa3a54609':
  ac3_parser: add a public function for parsing the data required by the demuxer

avpriv_ac3_parse_header() is left in place but without the
GetBitContext parameter, as the mov muxer requires a lot more fields
than just bitstream_id and frame_size from the AC3HeaderInfo struct.

Merged-by: James Almer <jamrial@gmail.com>