Make dither range symmetric about 0

Change-Id: I0c2eec60beb665c7dca9b9fddc00ac112a1ff2e1
Reviewed-on: https://skia-review.googlesource.com/24121
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
3 files changed