add yee's perceptual metric

experimental/skpdiff/SkPMetric.h

+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkPMetric_DEFINED
+#define SkPMetric_DEFINED
+
+#include "SkTDArray.h"
+
+#include "SkImageDiffer.h"
+
+/**
+ * An image differ that uses the pdiff image metric to compare images.
+ */
+class SkPMetric : public SkImageDiffer {
+public:
+    virtual const char* getName() SK_OVERRIDE;
+    virtual int queueDiff(SkBitmap* baseline, SkBitmap* test) SK_OVERRIDE;
+    virtual bool isFinished(int id) SK_OVERRIDE;
+    virtual double getResult(int id) SK_OVERRIDE;
+
+private:
+    struct QueuedDiff {
+        bool finished;
+        double result;
+    };
+
+    SkTDArray<QueuedDiff> fQueuedDiffs;
+
+    typedef SkImageDiffer INHERITED;
+};
+
+
+#endif