Update SkImageFilter's cache to handle SkSpecialImages and add unit test

include/core/SkImageFilter.h

Index: include/core/SkImageFilter.h
diff --git a/include/core/SkImageFilter.h b/include/core/SkImageFilter.h
index 29f6f7b760e2b2e27f9ab2dad92d9d8e0e2b5034..4f4d11d0165fdf204477a3e3c16c9dd577bfac3c 100644
--- a/include/core/SkImageFilter.h
+++ b/include/core/SkImageFilter.h
@@ -23,6 +23,7 @@ class SkBaseDevice;
 class SkBitmap;
 class SkColorFilter;
 struct SkIPoint;
+class SkSpecialImage;
 
 /**
  *  Base class for image filters. If one is installed in the paint, then
@@ -42,7 +43,9 @@ public:
         static Cache* Create(size_t maxBytes);
         static Cache* Get();
         virtual bool get(const Key& key, SkBitmap* result, SkIPoint* offset) const = 0;
+        virtual SkSpecialImage* get(const Key& key, SkIPoint* offset) const = 0;
         virtual void set(const Key& key, const SkBitmap& result, const SkIPoint& offset) = 0;
+        virtual void set(const Key& key, SkSpecialImage* image, const SkIPoint& offset) = 0;
         virtual void purge() {}
         virtual void purgeByKeys(const Key[], int) {}
     };