This patch adds support for encoding VP8 files with alpha. The alpha channel
is encoded separately and the output is placed in AVPacket's side_data. The
muxer then muxes it into the BlockAdditional element of the matroska container.
More details on spec here: http://goo.gl/wCP1ySigned-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>