Specialized scale down sample to 1 / 2 size adjust to match general purpose code which uses odd pixel (rounded up - nearest neighbor).
BUG=223
TEST=out\Debug\convert.exe -f 0 faces_640x480_P420.yuv face2_320x240_P420.yuv
R=johannkoenig@google.com

Review URL: https://webrtc-codereview.appspot.com/1583005

git-svn-id: http://libyuv.googlecode.com/svn/trunk@708 16f28f9a-4ce2-e073-06de-1de4eb20be90