Add SkCodec, including PNG implementation.

src/codec/SkCodec_libpng.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkCodec.h"
+#include "SkImageInfo.h"
+
+extern "C" {
+    // FIXME: I'd like to force all platforms to use the same decoder, but this
+    // means an extra dependency on Mac/Win.
+    #include "png.h"
+}
+
+class SkStream;
+
+class SkPngCodec : public SkCodec {
+public:
+    // Assumes IsPng was called and returned true.
+    static SkCodec* NewFromStream(SkStream*);
+    static bool IsPng(SkStream*);
+protected:
+    Result onGetPixels(const SkImageInfo&, void*, size_t, SkPMColor*, int*) SK_OVERRIDE;
+private:
+    png_structp             fPng_ptr;
+    png_infop               fInfo_ptr;
+
+    SkPngCodec(const SkImageInfo&, SkStream*, png_structp, png_infop);
+    ~SkPngCodec();
+
+    typedef SkCodec INHERITED;
+};