avcodec/libvpxdec: Adds decode support for formats other than 420

Handles decoding of new VP9 profiles 1-3 with different color sampling
and bit-depths.

For high bitdepth (profiles 2 and 3) support, we currently need to link
with the highbitdepth branch of libvpx with --enable-experimental
and --enable-vp9-high config options on. But eventually this branch will
be merged into master, whereafter to enable high bitdepth
support you will need to link with libvpx with configure option
--enable-vp9-highbitdepth on.

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