A ConvertFromI420 function.  Supports basic conversion with no clipping, inversion or rotation, for a subset of formats - most of the single plane formats,  all 3 plane formats and no 2 plane formats. (ie nv12)
BUG=none
TEST=none
Review URL: http://webrtc-codereview.appspot.com/329011

git-svn-id: http://libyuv.googlecode.com/svn/trunk@117 16f28f9a-4ce2-e073-06de-1de4eb20be90