Make unittests to 1280 pixels for simple planar tests, to get more realistic performance metrics than 256 pixels.
BUG=253
TEST=planar tests
R=nfullagar@google.com

Review URL: https://webrtc-codereview.appspot.com/1994004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@753 16f28f9a-4ce2-e073-06de-1de4eb20be90