update skpdiff visualization (image magnification with alpha mask)

tools/skpdiff/SkDiffContext.h

Index: tools/skpdiff/SkDiffContext.h
diff --git a/tools/skpdiff/SkDiffContext.h b/tools/skpdiff/SkDiffContext.h
index 20193b38514248e77ba6e7a34752cf299a413d14..9669ae0ad39e79250f6007c30a3bf47707e10f37 100644
--- a/tools/skpdiff/SkDiffContext.h
+++ b/tools/skpdiff/SkDiffContext.h
@@ -26,6 +26,12 @@ public:
     void setThreadCount(int threadCount) { fThreadCount = threadCount; }
 
     /**
+     * Creates the directory if it does not exist and uses it to store differences
+     * between images.
+     */
+    void setDifferenceDir(const SkString& directory);
+
+    /**
      * Sets the differs to be used in each diff. Already started diffs will not retroactively use
      * these.
      * @param differs An array of differs to use. The array is copied, but not the differs
@@ -106,6 +112,7 @@ private:
     };
 
     struct DiffRecord {
+        SkString           fCommonName;
         SkString           fDifferencePath;
         SkString           fBaselinePath;
         SkString               fTestPath;
@@ -121,6 +128,8 @@ private:
     SkImageDiffer** fDiffers;
     int fDifferCount;
     int fThreadCount;
+
+    SkString fDifferenceDir;
 };
 
 #endif