mlpdec: support major sync headers with optional extension blocks

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
mlpdec: support TrueHD streams with an Atmos substream

The fourth substream is being discarded, since its not raw audio data,
but an encoded Atmos stream which needs a specialized decoder.

Fixes decoding of the true hd stream from Transformers\ -\ Age\ of\ Extinction\ 2014\ 1080P-003.mkv

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