Add radial gradient stage.

Change-Id: Ie1f9640f5149f21bd8b3b864ff8b176232e1b0a9
Reviewed-on: https://skia-review.googlesource.com/15461
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>
6 files changed