Make SkImageFilter methods const.

SkImageFilter had some non-const methods that could all be made const.
This is a first step towards making SkImageFilter immutable.

BUG=skia:2097
R=mtklein@google.com, reed@google.com, robertphillips@google.com

Author: dominikg@chromium.org

Review URL: https://codereview.chromium.org/148883011

git-svn-id: http://skia.googlecode.com/svn/trunk@13330 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/effects/SkPictureImageFilter.h b/include/effects/SkPictureImageFilter.h
index ccda85d..eeaf1d2 100644
--- a/include/effects/SkPictureImageFilter.h
+++ b/include/effects/SkPictureImageFilter.h
@@ -31,7 +31,7 @@
     explicit SkPictureImageFilter(SkReadBuffer& buffer);
     virtual void flatten(SkWriteBuffer&) const SK_OVERRIDE;
     virtual bool onFilterImage(Proxy*, const SkBitmap& src, const SkMatrix&,
-                               SkBitmap* result, SkIPoint* offset) SK_OVERRIDE;
+                               SkBitmap* result, SkIPoint* offset) const SK_OVERRIDE;
 
 private:
     SkPicture* fPicture;