Implement intra-frame cacheing in image filters.

gm/imagefiltersbase.cpp

Index: gm/imagefiltersbase.cpp
diff --git a/gm/imagefiltersbase.cpp b/gm/imagefiltersbase.cpp
index b31fbebe605e63b84747cb013fc7bf887a585a77..2970ff285a70da22683b8bb5ab37aecd99a4c542 100644
--- a/gm/imagefiltersbase.cpp
+++ b/gm/imagefiltersbase.cpp
@@ -26,7 +26,7 @@ public:
 protected:
     FailImageFilter() : INHERITED(0) {}
     virtual bool onFilterImage(Proxy*, const SkBitmap& src, const Context&,
-                               SkBitmap* result, SkIPoint* offset) const {
+                               SkBitmap* result, SkIPoint* offset) const SK_OVERRIDE {
         return false;
     }
 
@@ -52,8 +52,9 @@ public:
 protected:
     IdentityImageFilter() : INHERITED(0) {}
     virtual bool onFilterImage(Proxy*, const SkBitmap& src, const Context&,
-                               SkBitmap* result, SkIPoint* offset) const {
+                               SkBitmap* result, SkIPoint* offset) const SK_OVERRIDE {
         *result = src;
+        offset->set(0, 0);
         return true;
     }