Add SkCanvasStack and update the Canvas utilities to use it.

src/utils/SkCanvasStack.h

Index: src/utils/SkCanvasStack.h
diff --git a/src/utils/SkCanvasStack.h b/src/utils/SkCanvasStack.h
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index 0000000000000000000000000000000000000000..3ca1dc214995010e641ba375f3c58e4e7d9bbb0d
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+++ b/src/utils/SkCanvasStack.h
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+
+/*
+ * 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 SkCanvasStack_DEFINED
+#define SkCanvasStack_DEFINED
+
+#include "SkNWayCanvas.h"
+#include "SkTArray.h"
+
+class SkCanvasStack : public SkNWayCanvas {
+public:
+    SkCanvasStack(int width, int height);
+    virtual ~SkCanvasStack();
+
+    void pushCanvas(SkCanvas* canvas, SkIPoint origin);
+    virtual void removeAll() SK_OVERRIDE;
+
+    /*
+     * The following add/remove canvas methods are overrides from SkNWayCanvas
+     * that do not make sense in the context of our CanvasStack, but since we
+     * can share most of the other implementation of NWay we override those
+     * methods to be no-ops.
+     */
+    virtual void addCanvas(SkCanvas*) SK_OVERRIDE {};
+    virtual void removeCanvas(SkCanvas*) SK_OVERRIDE {};
+
+    virtual void setMatrix(const SkMatrix& matrix) SK_OVERRIDE;
+    virtual bool clipRect(const SkRect&, SkRegion::Op, bool) SK_OVERRIDE;
+    virtual bool clipRRect(const SkRRect&, SkRegion::Op, bool) SK_OVERRIDE;
+    virtual bool clipPath(const SkPath&, SkRegion::Op, bool) SK_OVERRIDE;
+    virtual bool clipRegion(const SkRegion& deviceRgn,
+                            SkRegion::Op) SK_OVERRIDE;
+
+private:
+    struct CanvasData {
+        SkIPoint origin;
+        SkRegion requiredClip;
+    };
+
+    SkTArray<CanvasData> fCanvasData;
+
+    typedef SkNWayCanvas INHERITED;
+};
+
+#endif