tidy up dither stage

Using the float iota was just an expedient to write the stage...
this CL adds the U32 iota that dither really wants.

Change-Id: I7990b10afd0c5277186b6b8e730245d291bcef0c
Reviewed-on: https://skia-review.googlesource.com/17441
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
5 files changed