This patch implements support reading ALAC from Matroska files. The
only non-trivial thing about it is that only the ALAC magic cookie is
stored in Matroska's CodecPrivate element but not the "atom size",
"tag" and "tag version" fields that FFMPEG's ALAC decoder
expects. However, those are trivial to re-create.

Sample files are available:
http://www.bunkus.org/videotools/mkvtoolnix/samples/alac/alac-in-matroska.mka
and the CoreAudio file it was created from with today's mkvmerge:
http://www.bunkus.org/videotools/mkvtoolnix/samples/alac/alac-in-matroska-source.cafSigned-off-by: Michael Niedermayer <michaelni@gmx.at>