SkPDF: Implement Paeth predictor for images.

src/pdf/SkPngPredictors.h

Index: src/pdf/SkPngPredictors.h
diff --git a/src/pdf/SkPngPredictors.h b/src/pdf/SkPngPredictors.h
new file mode 100644
index 0000000000000000000000000000000000000000..5c1d4fbdf653eef21dae4d14114d902b09c5f2e6
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+++ b/src/pdf/SkPngPredictors.h
@@ -0,0 +1,24 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#ifndef SkPngPredictors_DEFINED
+#define SkPngPredictors_DEFINED
+
+#include "stdint.h"
+
+uint8_t SkPngEncode3(int width, const uint8_t* prev,
+                     const uint8_t* scanline, uint8_t* out);
+
+uint8_t SkPngEncode1(int width, const uint8_t* prev,
+                     const uint8_t* scanline, uint8_t* out);
+
+uint8_t SkPaethEncode3(int width, const uint8_t* prev,
+                       const uint8_t* scanline, uint8_t* out);
+
+uint8_t SkPaethEncode1(int width, const uint8_t* prev,
+                       const uint8_t* scanline, uint8_t* out);
+
+#endif  // SkPngPredictors_DEFINED