Implement SkGLContext swapBuffers with fence syncs

include/gpu/gl/GrGLInterface.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLInterface_DEFINED
 #define GrGLInterface_DEFINED
 
@@ skipping 12 matching lines @@
  *
  * The default interface is returned by GrGLDefaultInterface. This function's
  * implementation is platform-specific. Several have been provided, along with
  * an implementation that simply returns NULL.
  *
  * By defining GR_GL_PER_GL_CALL_IFACE_CALLBACK to 1 the client can specify a
  * callback function that will be called prior to each GL function call. See
  * comments in GrGLConfig.h
  */
 
+typedef void(*GrGLFuncPtr)();
+
 struct GrGLInterface;
 
 const GrGLInterface* GrGLDefaultInterface();
 
 /**
  * Creates a GrGLInterface for a "native" GL context (e.g. WGL on windows,
  * GLX on linux, AGL on Mac). The interface is only valid for the GL context
  * that is current when the interface is created.
  */
 const GrGLInterface* GrGLCreateNativeInterface();
@@ skipping 454 matching lines @@
 #if GR_GL_PER_GL_FUNC_CALLBACK
     GrGLInterfaceCallbackProc fCallback;
     GrGLInterfaceCallbackData fCallbackData;
 #endif
 
     // This exists for internal testing.
     virtual void abandon() const {}
 };
 
 #endif