change Y multiplier and bias to compensate for 257/256 which makes YToARGB exactly match float math.
Histogram Before
hist            -3      -2      -1      0       1       2       3
red             0       0       1809408 13140736        1827072 0       0
green           0       0       1679912 13471329        1625975 0       0
blue            168448  994816  1876480 10655488        1893376 1006336 182272
Histogram After
hist            -3      -2      -1      0       1       2       3
red             0       0       558848  15632128        586240  0       0
green           0       0       209907  16350588        216721  0       0
blue            14848   642816  1989376 11363328        2053120 695040  18688
BUG=394
TESTED=more stringent luma tests
R=brucedawson@google.com

Review URL: https://webrtc-codereview.appspot.com/38859004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1259 16f28f9a-4ce2-e073-06de-1de4eb20be90