Make SkImageFilter methods const.

include/core/SkImageFilterUtils.h

 /*
  * Copyright 2013 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkImageFilterUtils_DEFINED
 #define SkImageFilterUtils_DEFINED
 
@@ skipping 10 matching lines @@
     /**
      * Wrap the given texture in a texture-backed SkBitmap.
      */
     static bool WrapTexture(GrTexture* texture, int width, int height, SkBitmap* result);
 
     /**
      * Recursively evaluate the given filter on the GPU.  If filter is NULL,
      * this function returns src.  If the filter has no GPU implementation, it
      * will be processed in software and uploaded to the GPU.
      */
-    static bool GetInputResultGPU(SkImageFilter* filter, SkImageFilter::Proxy* proxy,
+    static bool GetInputResultGPU(const SkImageFilter* filter, SkImageFilter::Proxy* proxy,
                                   const SkBitmap& src, const SkMatrix& ctm, SkBitmap* result,
                                   SkIPoint* offset);
 };
 
 #endif
 
 #endif