Make SkImageFilter methods const.

src/effects/SkColorFilterImageFilter.cpp

 /*
  * 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.
  */
 
 #include "SkColorFilterImageFilter.h"
 #include "SkBitmap.h"
 #include "SkCanvas.h"
@@ skipping 83 matching lines @@
     buffer.writeFlattenable(fColorFilter);
 }
 
 SkColorFilterImageFilter::~SkColorFilterImageFilter() {
     SkSafeUnref(fColorFilter);
 }
 
 bool SkColorFilterImageFilter::onFilterImage(Proxy* proxy, const SkBitmap& source,
                                              const SkMatrix& matrix,
                                              SkBitmap* result,
-                                             SkIPoint* offset) {
+                                             SkIPoint* offset) const {
     SkBitmap src = source;
     SkIPoint srcOffset = SkIPoint::Make(0, 0);
     if (getInput(0) && !getInput(0)->filterImage(proxy, source, matrix, &src, &srcOffset)) {
         return false;
     }
 
     SkIRect bounds;
     src.getBounds(&bounds);
     bounds.offset(srcOffset);
     if (!this->applyCropRect(&bounds, matrix)) {
@@ skipping 20 matching lines @@
 bool SkColorFilterImageFilter::asColorFilter(SkColorFilter** filter) const {
     if (!cropRectIsSet()) {
         if (filter) {
             *filter = fColorFilter;
             fColorFilter->ref();
         }
         return true;
     }
     return false;
 }