Dither can bump color values above alpha (duh), or below
zero (duh), so clamp back to premul after dithering.
BUG=skia:6644,skia:6643
Change-Id: Ida107e866380e06130af0d01467117bca929ba44
Reviewed-on: https://skia-review.googlesource.com/17070
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
