avcodec/ac3: move new field to the end of AC3HeaderInfo

This structure is used in the interface between libs and thus cannot have
fields added in the middle without major bump

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/ac3_parser: add avpriv_ac3_parse_header2() and use it in libavcodec

The new function has the ability to allocate the structure, allowing it to grow
without needing major bumps

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/ac3dec: Use avpriv_ac3_parse_header2()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>