Change Sobel to use JPeg Luma calculation instead of extracting G channel.  Using luma produces a better sobel that respects all 3 channels of RGB.  Historically the G channel was used to improve performance, and because the luma of I420 is a constrained range, hurting quality.  Using the JPeg variation of YUV, the luma is more accurate, including cross platform, better optimized for AVX2 and odd widths, and full range.
BUG=444
TESTED=ARGBSobelXY_Opt
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/57479004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1414 16f28f9a-4ce2-e073-06de-1de4eb20be90