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/src/effects/SkTileImageFilter.cpp b/src/effects/SkTileImageFilter.cpp
index f0b7fda..f17e2ea 100644
--- a/src/effects/SkTileImageFilter.cpp
+++ b/src/effects/SkTileImageFilter.cpp
@@ -17,7 +17,7 @@
 #include "SkValidationUtils.h"
 
 bool SkTileImageFilter::onFilterImage(Proxy* proxy, const SkBitmap& src, const SkMatrix& ctm,
-                                      SkBitmap* dst, SkIPoint* offset) {
+                                      SkBitmap* dst, SkIPoint* offset) const {
     SkBitmap source = src;
     SkImageFilter* input = getInput(0);
     SkIPoint srcOffset = SkIPoint::Make(0, 0);