This moves where dither happens in CPU rendering to
more closely match where it happens in GPU rendering.

Bug: skia:10000
Change-Id: I184fd73599129596f3896ce716366f65fce60348
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2219071
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Cr-Commit-Position: refs/heads/master@{#772699}