lowp: lighten, difference, exclusion

Change-Id: I5773cf831c7e41a932bee1f2c6830085fb7db025
Reviewed-on: https://skia-review.googlesource.com/33764
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@google.com>
4 files changed