Go from a 1x2 to a 2x2 dither cell, and change/simplify the logic for how we
compute the "dithered" version of a color to just a graduated fixed-point-round.
Also, add this new dither to conical and sweep, which before had no dithering.

Disabled for now using SK_IGNORE_GRADIENT_DITHER_FIX. Will enable this and
and rebaseline skia.

http://code.google.com/p/skia/issues/detail?id=1098
Review URL: https://codereview.appspot.com/7248046

git-svn-id: http://skia.googlecode.com/svn/trunk@7549 2bbb7eff-a529-9590-31e7-b0007b416f81
6 files changed