dither stage

I think we can dither generically as a pipeline stage.

I'm not married to where the dither happens, or the implementation,
which is mostly cribbed from
https://en.wikipedia.org/wiki/Ordered_dithering.

BUG=skia:3302,skia:6224

Change-Id: If7f6b22a523ca0b34cb03c0aa97b6734c34e0133
Reviewed-on: https://skia-review.googlesource.com/15161
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
6 files changed