Make SkImageFilter methods const.

src/effects/SkRectShaderImageFilter.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 "SkRectShaderImageFilter.h"
 #include "SkBitmap.h"
 #include "SkCanvas.h"
@@ skipping 36 matching lines @@
 }
 
 SkRectShaderImageFilter::~SkRectShaderImageFilter() {
     SkSafeUnref(fShader);
 }
 
 bool SkRectShaderImageFilter::onFilterImage(Proxy* proxy,
                                             const SkBitmap& source,
                                             const SkMatrix& ctm,
                                             SkBitmap* result,
-                                            SkIPoint* offset) {
+                                            SkIPoint* offset) const {
     SkIRect bounds;
     source.getBounds(&bounds);
     if (!this->applyCropRect(&bounds, ctm)) {
         return false;
     }
 
     SkAutoTUnref<SkBaseDevice> device(proxy->createDevice(bounds.width(),
                                                           bounds.height()));
     if (NULL == device.get()) {
         return false;
     }
     SkCanvas canvas(device.get());
     SkPaint paint;
     paint.setShader(fShader);
     SkMatrix matrix;
     matrix.setTranslate(-SkIntToScalar(bounds.fLeft), -SkIntToScalar(bounds.fTop));
     fShader->setLocalMatrix(matrix);
     SkRect rect = SkRect::MakeWH(SkIntToScalar(bounds.width()), SkIntToScalar(bounds.height()));
     canvas.drawRect(rect, paint);
     *result = device.get()->accessBitmap(false);
     offset->fX = bounds.fLeft;
     offset->fY = bounds.fTop;
     return true;
 }