Make SkImageFilter methods const.

src/effects/SkBitmapSource.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 "SkBitmapSource.h"
 #include "SkDevice.h"
 #include "SkCanvas.h"
@@ skipping 25 matching lines @@
 }
 
 void SkBitmapSource::flatten(SkWriteBuffer& buffer) const {
     this->INHERITED::flatten(buffer);
     fBitmap.flatten(buffer);
     buffer.writeRect(fSrcRect);
     buffer.writeRect(fDstRect);
 }
 
 bool SkBitmapSource::onFilterImage(Proxy* proxy, const SkBitmap&, const SkMatrix& matrix,
-                                   SkBitmap* result, SkIPoint* offset) {
+                                   SkBitmap* result, SkIPoint* offset) const {
     SkRect bounds, dstRect;
     fBitmap.getBounds(&bounds);
     matrix.mapRect(&dstRect, fDstRect);
     if (fSrcRect == bounds && dstRect == bounds) {
         // No regions cropped out or resized; return entire bitmap.
         *result = fBitmap;
         offset->fX = offset->fY = 0;
         return true;
     }
     SkIRect dstIRect;
@@ skipping 25 matching lines @@
 
 void SkBitmapSource::computeFastBounds(const SkRect&, SkRect* dst) const {
     *dst = fDstRect;
 }
 
 bool SkBitmapSource::onFilterBounds(const SkIRect& src, const SkMatrix& ctm,
                                     SkIRect* dst) const {
     *dst = src;
     return true;
 }