add skpdiff tool to compare bitmaps

- start framework for pluggable algorithms
- implement simple number of pixels different OpenCL algo

R=djsollen@google.com, bsalomon@google.com, jvanverth@google.com

Author: zachr@google.com

Review URL: https://chromiumcodereview.appspot.com/16284007

git-svn-id: http://skia.googlecode.com/svn/trunk@9616 2bbb7eff-a529-9590-31e7-b0007b416f81
10 files changed