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
new file mode 100644
index 0000000000000000000000000000000000000000..2e2c9b21fa3aaea289d14a1747d4bfe80d9a77da
--- /dev/null
+++ b/src/utils/SkCanvasStack.h
@@ -0,0 +1,52 @@
+
+/*
+ * 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, const SkIPoint& origin);
+    virtual void removeAll() SK_OVERRIDE;

[reed1, 2013/09/04 15:00:00]

Do we need to expose this guy for Clank, or can we mark it as "hidden" like
addCanvas/removeCanvas?

+
+    /*
+     * 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 {};

[reed1, 2013/09/04 15:00:00]

SkASSERT(!"don't call addCanvas on a CanvasStack"); ???

+    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:
+    void clipToZOrderedBounds();
+
+    struct CanvasData {
+        SkIPoint origin;
+        SkRegion requiredClip;
+    };
+
+    SkTArray<CanvasData> fCanvasData;
+
+    typedef SkNWayCanvas INHERITED;
+};
+
+#endif