BUG=292
TESTED=BackgroundOverlayKernelTest.ProcessVideo_ForegroundBackground

Review URL: https://webrtc-codereview.appspot.com/5379004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@905 16f28f9a-4ce2-e073-06de-1de4eb20be90