Make SkImageFilter methods const.

gm/imagefiltersgraph.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm.h"
 
 #include "SkArithmeticMode.h"
@@ skipping 11 matching lines @@
 #include "SkXfermodeImageFilter.h"
 
 // More closely models how Blink's OffsetFilter works as of 10/23/13. SkOffsetImageFilter doesn't
 // perform a draw and this one does.
 class SimpleOffsetFilter : public SkImageFilter {
 public:
     SimpleOffsetFilter(SkScalar dx, SkScalar dy, SkImageFilter* input)
     : SkImageFilter(input), fDX(dx), fDY(dy) {}
 
     virtual bool onFilterImage(Proxy* proxy, const SkBitmap& src, const SkMatrix& ctm,
-                               SkBitmap* dst, SkIPoint* offset) SK_OVERRIDE {
+                               SkBitmap* dst, SkIPoint* offset) const SK_OVERRIDE {
         SkBitmap source = src;
         SkImageFilter* input = getInput(0);
         SkIPoint srcOffset = SkIPoint::Make(0, 0);
         if (NULL != input && !input->filterImage(proxy, src, ctm, &source, &srcOffset)) {
             return false;
         }
 
         SkIRect bounds;
         source.getBounds(&bounds);
 
@@ skipping 163 matching lines @@
 private:
     typedef GM INHERITED;
     SkBitmap fBitmap;
     bool fInitialized;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 static skiagm::GM* MyFactory(void*) { return new ImageFiltersGraphGM; }
 static skiagm::GMRegistry reg(MyFactory);