SkPDF: Implement Paeth predictor for images.

src/pdf/SkPngPredictors.h

+/*
+ * 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