Bug fix for I420ToI400 and unittest all I420To packed formats.  Most import of the new I420 tests is I420ToYUY2.
BUG=none
TEST=libyuvTest.I420ToI400_OptVsC
Review URL: https://webrtc-codereview.appspot.com/767004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@331 16f28f9a-4ce2-e073-06de-1de4eb20be90