Scale up point use step / 2 as initial coordinate, which is more symetric and matches ffmpeg exactly.
BUG=232
TEST=convert.exe -f 0 faces_352x288_P420.yuv faces_640x480_P420.yuv
R=ryanpetrie@google.com

Review URL: https://webrtc-codereview.appspot.com/1580005

git-svn-id: http://libyuv.googlecode.com/svn/trunk@706 16f28f9a-4ce2-e073-06de-1de4eb20be90