BUG=407
TESTED=ARGBToRGB565Dither unittest
R=brucedawson@google.com, harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/44519004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1309 16f28f9a-4ce2-e073-06de-1de4eb20be90