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/SkTestImageFilters.cpp b/src/effects/SkTestImageFilters.cpp
index 96226f5..6927932 100755
--- a/src/effects/SkTestImageFilters.cpp
+++ b/src/effects/SkTestImageFilters.cpp
@@ -23,7 +23,7 @@
 
 bool SkDownSampleImageFilter::onFilterImage(Proxy* proxy, const SkBitmap& src,
                                             const SkMatrix&,
-                                            SkBitmap* result, SkIPoint*) {
+                                            SkBitmap* result, SkIPoint*) const {
     SkScalar scale = fScale;
     if (scale > SK_Scalar1 || scale <= 0) {
         return false;