Make SkImageFilter methods const.

include/effects/SkMergeImageFilter.h

 /*
  * Copyright 2012 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 SkMergeImageFilter_DEFINED
 #define SkMergeImageFilter_DEFINED
 
@@ skipping 11 matching lines @@
                        const CropRect* cropRect = NULL);
     virtual ~SkMergeImageFilter();
 
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkMergeImageFilter)
 
 protected:
     SkMergeImageFilter(SkReadBuffer& buffer);
     virtual void flatten(SkWriteBuffer&) const SK_OVERRIDE;
 
     virtual bool onFilterImage(Proxy*, const SkBitmap& src, const SkMatrix&,
-                               SkBitmap* result, SkIPoint* loc) SK_OVERRIDE;
+                               SkBitmap* result, SkIPoint* loc) const SK_OVERRIDE;
 private:
     uint8_t*            fModes; // SkXfermode::Mode
 
     // private storage, to avoid dynamically allocating storage for our copy
     // of the modes (unless the count is so large we can't fit).
     intptr_t    fStorage[16];
 
     void initAllocModes();
     void initModes(const SkXfermode::Mode []);
 
     typedef SkImageFilter INHERITED;
 };
 
 #endif