Alpha blend 2 sources and store to a destination.  Useful for A under B blending.
BUG=none
TEST=none
Review URL: https://webrtc-codereview.appspot.com/472005

git-svn-id: http://libyuv.googlecode.com/svn/trunk@233 16f28f9a-4ce2-e073-06de-1de4eb20be90
8 files changed