more lowp blend modes

Change-Id: Id62e989d4278f273c040b159ed4d2fd6a2f209e0
Reviewed-on: https://skia-review.googlesource.com/18627
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed