When scaling a 1x1, dx/dy is 0.  Switch to point sample
BUG=none
TEST=none
R=dingkai@google.com

Review URL: https://webrtc-codereview.appspot.com/1582007

git-svn-id: http://libyuv.googlecode.com/svn/trunk@714 16f28f9a-4ce2-e073-06de-1de4eb20be90
3 files changed