Improve float-based dither logic

This code simulates the integer-based ordered-dither using step/mod
with only floating point values. Produces similar results.

R=bsalomon@google.com

Bug: skia:4430
Change-Id: I1406f751f0ddd6bfd14e532dfb4efc0bb5784992
Reviewed-on: https://skia-review.googlesource.com/28942
Commit-Queue: Eric Karl <ericrk@chromium.org>
Reviewed-by: Brian Salomon <bsalomon@google.com>
2 files changed