Dynamically create stencil buffer when needed.

src/gpu/GrRenderTargetPriv.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.
+ */
+
+#ifndef GrRenderTargetPriv_DEFINED
+#define GrRenderTargetPriv_DEFINED
+
+#include "GrRenderTarget.h"
+
+/** Class that adds methods to GrRenderTarget that are only intended for use internal to Skia.
+    This class is purely a privileged window into GrRenderTarget. It should never have additional
+    data members or virtual methods. */
+class GrRenderTargetPriv {
+public:
+    /**
+     * GrStencilBuffer is not part of the public API.
+     */
+    GrStencilBuffer* getStencilBuffer() const { return fRenderTarget->fStencilBuffer; }
+
+    /**
+     * If this render target already has a stencil buffer, return it. Otherwise attempt to attach
+     * one.
+     */
+    GrStencilBuffer* attachStencilBuffer() const;
+
+    void didAttachStencilBuffer(GrStencilBuffer*);
+
+private:
+    explicit GrRenderTargetPriv(GrRenderTarget* renderTarget) : fRenderTarget(renderTarget) {}
+    GrRenderTargetPriv(const GrRenderTargetPriv&) {} // unimpl
+    GrRenderTargetPriv& operator=(const GrRenderTargetPriv&); // unimpl
+
+    // No taking addresses of this type.
+    const GrRenderTargetPriv* operator&() const;
+    GrRenderTargetPriv* operator&();
+
+    GrRenderTarget* fRenderTarget;
+        
+    friend class GrRenderTarget; // to construct/copy this type.
+};
+
+inline GrRenderTargetPriv GrRenderTarget::renderTargetPriv() { return GrRenderTargetPriv(this); }
+
+inline const GrRenderTargetPriv GrRenderTarget::renderTargetPriv () const {
+    return GrRenderTargetPriv(const_cast<GrRenderTarget*>(this));
+}
+
+#endif