update skpdiff visualization (image magnification with alpha mask)

tools/skpdiff/SkDifferentPixelsMetric.h

Index: tools/skpdiff/SkDifferentPixelsMetric.h
diff --git a/tools/skpdiff/SkDifferentPixelsMetric.h b/tools/skpdiff/SkDifferentPixelsMetric.h
index 38fa5ac55c826e59a02b67a82d2d199791b914e1..614f92035672b214ac7b886449fca6742acf4eb3 100644
--- a/tools/skpdiff/SkDifferentPixelsMetric.h
+++ b/tools/skpdiff/SkDifferentPixelsMetric.h
@@ -27,13 +27,17 @@ class SkDifferentPixelsMetric :
     public SkImageDiffer {
 #endif
 public:
+    SkDifferentPixelsMetric() : fPOIAlphaMask(false) {}
+
     virtual const char* getName() SK_OVERRIDE;
+    virtual bool enablePOIAlphaMask() SK_OVERRIDE;
     virtual int queueDiff(SkBitmap* baseline, SkBitmap* test) SK_OVERRIDE;
     virtual void deleteDiff(int id) SK_OVERRIDE;
     virtual bool isFinished(int id) SK_OVERRIDE;
     virtual double getResult(int id) SK_OVERRIDE;
     virtual int getPointsOfInterestCount(int id) SK_OVERRIDE;
     virtual SkIPoint* getPointsOfInterest(int id) SK_OVERRIDE;
+    virtual SkBitmap* getPointsOfInterestAlphaMask(int id) SK_OVERRIDE;
 
 protected:
 #if SK_SUPPORT_OPENCL
@@ -41,8 +45,9 @@ protected:
 #endif
 
 private:
-    struct QueuedDiff;
+    bool fPOIAlphaMask;
 
+    struct QueuedDiff;
     SkTDArray<QueuedDiff> fQueuedDiffs;
 
 #if SK_SUPPORT_OPENCL