add readPixels to SkImage

src/image/SkReadPixelsRec.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkReadPixelsRec_DEFINED
+#define SkReadPixelsRec_DEFINED
+
+#include "SkImageInfo.h"
+
+/**
+ *  Helper class to package and trim the parameters passed to readPixels()
+ */
+struct SkReadPixelsRec {
+    SkReadPixelsRec(const SkImageInfo& info, void* pixels, size_t rowBytes, int x, int y)
+        : fPixels(pixels)
+        , fRowBytes(rowBytes)
+        , fInfo(info)
+        , fX(x)
+        , fY(y)
+    {}
+
+    void*       fPixels;
+    size_t      fRowBytes;
+    SkImageInfo fInfo;
+    int         fX;
+    int         fY;
+
+    /*
+     *  On true, may have modified its fields (except fRowBytes) to make it a legal subset
+     *  of the specified src width/height.
+     *
+     *  On false, leaves self unchanged, but indicates that it does not overlap src, or
+     *  is not valid (e.g. bad fInfo) for readPixels().
+     */
+    bool trim(int srcWidth, int srcHeight);
+};
+
+#endif