Make I420ToARGB1555, I420ToARGB4444 and I420ToRGB565 call a single function per row, which may be implemented in assembly, but for now the function calls 2 functions internally.
BUG=none
TEST=none
Review URL: https://webrtc-codereview.appspot.com/965006

git-svn-id: http://libyuv.googlecode.com/svn/trunk@459 16f28f9a-4ce2-e073-06de-1de4eb20be90