Add base effect for gpu 2pt conical gradients

This is in preparation to add more "sub-effects" for the various fast path cases for 2 pt conical gradients

BUG=skia:
R=jvanverth@google.com, robertphillips@google.com, bsalomon@google.com

Author: egdaniel@google.com

Review URL: https://codereview.chromium.org/224083002

git-svn-id: http://skia.googlecode.com/svn/trunk@14059 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed