Add sweep gradient to SkRasterPipeline

This is a prototype. I have remove the tiling, 
but maybe I should add it back in.

I thinking about factoring out the common code with 
linear gradient in its own CL.

I think radial gradient will be very close to this.

Change-Id: I1dfcb4f944138ee623afdf10b2a8befde797c604
Reviewed-on: https://skia-review.googlesource.com/13766
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>
6 files changed