update skpdiff visualization (image magnification with alpha mask)

tools/skpdiff/SkImageDiffer.h

Index: tools/skpdiff/SkImageDiffer.h
diff --git a/tools/skpdiff/SkImageDiffer.h b/tools/skpdiff/SkImageDiffer.h
index 6c570cbb8e41bc3d8437937047f55396cd262258..b46ba65b2b948e2e5c1504ac6b9f111ad6f2ab02 100644
--- a/tools/skpdiff/SkImageDiffer.h
+++ b/tools/skpdiff/SkImageDiffer.h
@@ -37,6 +37,11 @@ public:
     virtual bool requiresOpenCL() { return false; }
 
     /**
+     *
+     */
+    virtual void enablePOIAlphaMask() { fPOIAlphaMask = true; }

[epoger, 2013/10/21 15:02:53]

Related to my comments above about fPOIAlphaMask: maybe instead, match
getPointsOfInterestAlphaMask below:

virtual void enablePOIAlphaMask() { /* do nothing */ }

and override enablePOIAlphaMask() in SkDifferentPixelsMetric class

[djsollen, 2013/11/06 16:02:30]

Done.

+
+    /**
      * Wraps a call to queueDiff by loading the given filenames into SkBitmaps
      * @param  baseline The file path of the baseline image
      * @param  test     The file path of the test image
@@ -87,10 +92,15 @@ public:
      */
     virtual SkIPoint* getPointsOfInterest(int id) = 0;
 
+    /*
+     *
+     */
+    virtual SkBitmap* getPointsOfInterestAlphaMask(int id) { return NULL; }
 
 
 protected:
     bool fIsGood;
+    bool fPOIAlphaMask;
 };