ac3: read and store the Dolby Surround mode flag.
ac3dec: store the bitstream ID in the AC3DecodeContext.

This is needed to detect and parse the alternate bitstream syntax.
ac3dec: read and store the Dolby Surround EX mode flag.
ac3dec: read and store the Dolby Headphone mode flag.
eac3dec: read and store the Dolby Surround mode flag.
eac3dec: read and store the Dolby Surround EX mode flag.
eac3dec: read and store the Dolby Headphone mode flag.