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/SkDisplacementMapEffect.cpp b/src/effects/SkDisplacementMapEffect.cpp
index 555f795..a563a6e 100644
--- a/src/effects/SkDisplacementMapEffect.cpp
+++ b/src/effects/SkDisplacementMapEffect.cpp
@@ -195,10 +195,10 @@
                                             const SkBitmap& src,
                                             const SkMatrix& ctm,
                                             SkBitmap* dst,
-                                            SkIPoint* offset) {
+                                            SkIPoint* offset) const {
     SkBitmap displ = src, color = src;
-    SkImageFilter* colorInput = getColorInput();
-    SkImageFilter* displInput = getDisplacementInput();
+    const SkImageFilter* colorInput = getColorInput();
+    const SkImageFilter* displInput = getDisplacementInput();
     SkIPoint colorOffset = SkIPoint::Make(0, 0), displOffset = SkIPoint::Make(0, 0);
     if ((colorInput && !colorInput->filterImage(proxy, src, ctm, &color, &colorOffset)) ||
         (displInput && !displInput->filterImage(proxy, src, ctm, &displ, &displOffset))) {
@@ -348,7 +348,7 @@
 };
 
 bool SkDisplacementMapEffect::filterImageGPU(Proxy* proxy, const SkBitmap& src, const SkMatrix& ctm,
-                                             SkBitmap* result, SkIPoint* offset) {
+                                             SkBitmap* result, SkIPoint* offset) const {
     SkBitmap colorBM;
     SkIPoint colorOffset = SkIPoint::Make(0, 0);
     if (!SkImageFilterUtils::GetInputResultGPU(getColorInput(), proxy, src, ctm, &colorBM,