Change point down sampling to x = dx / 2 which matches ffmpeg and is lossless on up and then down sample.
BUG=232
TEST=convert.exe -f 0 faces_640x480_P420.yuv face2_352x288_P420.yuv
R=johannkoenig@google.com, ryanpetrie@google.com

Review URL: https://webrtc-codereview.appspot.com/1581005

git-svn-id: http://libyuv.googlecode.com/svn/trunk@707 16f28f9a-4ce2-e073-06de-1de4eb20be90