make OpenCL optional for skpdiff

- add CPU based different pixels metric

R=djsollen@google.com

Review URL: https://codereview.chromium.org/19374006

git-svn-id: http://skia.googlecode.com/svn/trunk@10127 2bbb7eff-a529-9590-31e7-b0007b416f81
13 files changed