Make SkImageFilter methods const.

src/effects/SkComposeImageFilter.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmap.h"
 #include "SkComposeImageFilter.h"
 #include "SkReadBuffer.h"
 #include "SkWriteBuffer.h"
 
 SkComposeImageFilter::~SkComposeImageFilter() {
 }
 
 bool SkComposeImageFilter::onFilterImage(Proxy* proxy,
                                          const SkBitmap& src,
                                          const SkMatrix& ctm,
                                          SkBitmap* result,
-                                         SkIPoint* offset) {
+                                         SkIPoint* offset) const {
     SkImageFilter* outer = getInput(0);
     SkImageFilter* inner = getInput(1);
 
     if (!outer && !inner) {
         return false;
     }
 
     if (!outer || !inner) {
         return (outer ? outer : inner)->filterImage(proxy, src, ctm, result, offset);
     }
@@ skipping 18 matching lines @@
     }
 
     SkIRect tmp;
     return inner->filterBounds(src, ctm, &tmp) &&
            outer->filterBounds(tmp, ctm, dst);
 }
 
 SkComposeImageFilter::SkComposeImageFilter(SkReadBuffer& buffer)
   : INHERITED(2, buffer) {
 }