subsample yuy2 dest
subsample yuy2 dest

BUG=455
TESTED=out\release\libyuv_unittest.exe --gtest_catch_exceptions=0 --gtest_filter=*ARGBToYUY2*
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/58429004.

lyx2014/yuv,lemenkov/libyuv,lemenkov/libyuv,yarrcc/libyuv-ios,lyx2014/yuv,lyx2014/libyuv_free,lyx2014/yuv,lyx2014/libyuv_free,lyx2014/yuv,lemenkov/libyuv,disigma/yuv,lyx2014/libyuv_free,yarrcc/libyuv-ios,yarrcc/libyuv-ios,disigma/yuv,disigma/yuv,yarrcc/libyuv-ios,lemenkov/libyuv,lyx2014/libyuv_free