This patch adds support for muxing VP8 Alpha Files. The Alpha channel data is
placed in BlockAdditional element of the matroska container. More information
& exact spec on how this is implemented can be found here: http://goo.gl/wCP1ySigned-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>