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
diff --git a/Makefile b/Makefile
index 1a0e5a5..0bb3b70 100644
--- a/Makefile
+++ b/Makefile
@@ -48,7 +48,8 @@
                  SkiaAndroidApp \
                  skia_lib \
                  tests \
-                 tools
+                 tools \
+                 skpdiff
 
 # Default target.  This must be listed before all other targets.
 .PHONY: default