Make SkImageFilter methods const.

src/effects/SkPictureImageFilter.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 "SkPictureImageFilter.h"
 #include "SkDevice.h"
 #include "SkCanvas.h"
@@ skipping 27 matching lines @@
     buffer.readRect(&fRect);
 }
 
 void SkPictureImageFilter::flatten(SkWriteBuffer& buffer) const {
     this->INHERITED::flatten(buffer);
     // FIXME: flatten picture here.
     buffer.writeRect(fRect);
 }
 
 bool SkPictureImageFilter::onFilterImage(Proxy* proxy, const SkBitmap&, const SkMatrix& matrix,
-                                   SkBitmap* result, SkIPoint* offset) {
+                                   SkBitmap* result, SkIPoint* offset) const {
     if (!fPicture) {
         offset->fX = offset->fY = 0;
         return true;
     }
 
     SkRect floatBounds;
     SkIRect bounds;
     matrix.mapRect(&floatBounds, fRect);
     floatBounds.roundOut(&bounds);
 
@@ skipping 12 matching lines @@
 
     canvas.translate(-SkIntToScalar(bounds.fLeft), -SkIntToScalar(bounds.fTop));
     canvas.concat(matrix);
     canvas.drawPicture(*fPicture);
 
     *result = device.get()->accessBitmap(false);
     offset->fX = bounds.fLeft;
     offset->fY = bounds.fTop;
     return true;
 }