Fix alpha textures in NV ES3 contexts on Windows

src/gpu/gl/GrGLContext.h

 /*
  * 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 GrGLContext_DEFINED
 #define GrGLContext_DEFINED
@@ skipping 47 matching lines @@
     bool isChromium() const { return fIsChromium; }
     const GrGLCaps* caps() const { return fGLCaps.get(); }
     GrGLCaps* caps() { return fGLCaps; }
     bool hasExtension(const char* ext) const {
         if (!this->isInitialized()) {
             return false;
         }
         return fInterface->hasExtension(ext);
     }
 
+    const GrGLExtensions& extensions() const { return fInterface->fExtensions; }
+
     /**
      * Reset the information
      */
     void reset();
 
 protected:
     SkAutoTUnref<const GrGLInterface>   fInterface;
     GrGLVersion                         fGLVersion;
     GrGLSLGeneration                    fGLSLGeneration;
     GrGLVendor                          fVendor;
@@ skipping 23 matching lines @@
         return *this;
     }
 
     const GrGLInterface* interface() const { return fInterface.get(); }
 
 private:
     typedef GrGLContextInfo INHERITED;
 };
 
 #endif