implement 4f for composeshader

Bug: skia:
Change-Id: I7ad0077345f7589528c89cd84d252539a1df6614
Reviewed-on: https://skia-review.googlesource.com/16703
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed