tidy up dither stage

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>