add a GM to demo dither

Change-Id: Idbed35f2c5a1420b66b68725761a7ceea8cdd3ba
Reviewed-on: https://skia-review.googlesource.com/66461
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2 files changed