Make SkImageFilter methods const.

src/effects/SkResizeImageFilter.cpp

 /*
  * Copyright 2013 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 "SkResizeImageFilter.h"
 #include "SkBitmap.h"
 #include "SkCanvas.h"
@@ skipping 26 matching lines @@
     buffer.writeInt(fFilterLevel);
 }
 
 SkResizeImageFilter::~SkResizeImageFilter() {
 }
 
 bool SkResizeImageFilter::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;
     }
 
     SkRect dstRect;
     SkIRect srcBounds, dstBounds;
     src.getBounds(&srcBounds);
     srcBounds.offset(srcOffset);
@@ skipping 15 matching lines @@
     paint.setXfermodeMode(SkXfermode::kSrc_Mode);
     paint.setFilterLevel(fFilterLevel);
     canvas.concat(dstMatrix);
     canvas.drawBitmap(src, srcRect.left(), srcRect.top(), &paint);
 
     *result = device.get()->accessBitmap(false);
     offset->fX = dstBounds.fLeft;
     offset->fY = dstBounds.fTop;
     return true;
 }