Dynamically create stencil buffer when needed.

src/gpu/GrSurfacePriv.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 GrSurfacePriv_DEFINED
 #define GrSurfacePriv_DEFINED
 
@@ skipping 16 matching lines @@
      * Write the contents of the surface to a PNG. Returns true if successful.
      * @param filename      Full path to desired file
      */
     bool savePixels(const char* filename) { return fSurface->savePixels(filename); }
 
     bool hasPendingRead() const { return fSurface->hasPendingRead(); }
     bool hasPendingWrite() const { return fSurface->hasPendingWrite(); }
     bool hasPendingIO() const { return fSurface->hasPendingIO(); }
 
 private:
-    GrSurfacePriv(GrSurface* surface) : fSurface(surface) { }
-    GrSurfacePriv(const GrSurfacePriv& that) : fSurface(that.fSurface) { }
-    GrSurfacePriv& operator=(const GrSurface&); // unimpl
+    explicit GrSurfacePriv(GrSurface* surface) : fSurface(surface) {}
+    GrSurfacePriv(const GrSurfacePriv&); // unimpl
+    GrSurfacePriv& operator=(const GrSurfacePriv&); // unimpl
 
     // No taking addresses of this type.
     const GrSurfacePriv* operator&() const;
     GrSurfacePriv* operator&();
 
     GrSurface* fSurface;
 
     friend class GrSurface; // to construct/copy this type.
 };
 
 inline GrSurfacePriv GrSurface::surfacePriv() { return GrSurfacePriv(this); }
 
 inline const GrSurfacePriv GrSurface::surfacePriv() const {
     return GrSurfacePriv(const_cast<GrSurface*>(this));
 }
 
 #endif