Make SkImageFilter methods const.

src/effects/SkOffsetImageFilter.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 "SkOffsetImageFilter.h"
 #include "SkBitmap.h"
 #include "SkCanvas.h"
 #include "SkDevice.h"
 #include "SkReadBuffer.h"
 #include "SkWriteBuffer.h"
 #include "SkMatrix.h"
 #include "SkPaint.h"
 
 bool SkOffsetImageFilter::onFilterImage(Proxy* proxy, const SkBitmap& source,
                                         const SkMatrix& matrix,
                                         SkBitmap* result,
-                                        SkIPoint* offset) {
+                                        SkIPoint* offset) const {
     SkImageFilter* input = getInput(0);
     SkBitmap src = source;
     SkIPoint srcOffset = SkIPoint::Make(0, 0);
 #ifdef SK_DISABLE_OFFSETIMAGEFILTER_OPTIMIZATION
     if (false) {
 #else
     if (!cropRectIsSet()) {
 #endif
         if (input && !input->filterImage(proxy, source, matrix, &src, &srcOffset)) {
             return false;
@@ skipping 66 matching lines @@
                                          const CropRect* cropRect) : INHERITED(input, cropRect) {
     fOffset.set(dx, dy);
 }
 
 SkOffsetImageFilter::SkOffsetImageFilter(SkReadBuffer& buffer)
   : INHERITED(1, buffer) {
     buffer.readPoint(&fOffset);
     buffer.validate(SkScalarIsFinite(fOffset.fX) &&
                     SkScalarIsFinite(fOffset.fY));
 }