[External patch] Source-over support for SkLumaXfermode.

This is a patch by Andrei Parvu <parvu@adobe.com> (Adobe CLA signer).

Original CL/review: https://codereview.chromium.org/24078006/

GM:lumamode will need rebaselining after landing this.

---

  In order to use CSS luminance masking, we need to be able to create an
  instance of SkLumaXfermode which can receive a kSrcOver mode, and applies
  that mode after converting the source using the luminance-to-alpha
  coefficients.

BUG=289420
R=reed@google.com

Review URL: https://codereview.chromium.org/23710053

git-svn-id: http://skia.googlecode.com/svn/trunk@11312 2bbb7eff-a529-9590-31e7-b0007b416f81
6 files changed