Code sourced from libvpx.
  http://www.webmproject.org/code/#libvpx_the_vp8_codec_sdk
Review URL: http://webrtc-codereview.appspot.com/267004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@72 16f28f9a-4ce2-e073-06de-1de4eb20be90