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

src/core/SkSpecialImage.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file
  */
 
 #ifndef SkSpecialImage_DEFINED
 #define SkSpecialImage_DEFINED
 
@@ skipping 23 matching lines @@
  *      - it can't be drawn tiled
  *      - it can't be drawn with MIPMAPs
  * It is similar to SkImage in that it abstracts how the pixels are stored/represented.
  *
  * Note: the contents of the backing storage outside of the subset rect are undefined.
  */
 class SkSpecialImage : public SkRefCnt {
 public:
     int width() const { return fSubset.width(); }
     int height() const { return fSubset.height(); }
+    const SkIRect& subset() const { return fSubset; }
+
     uint32_t uniqueID() const { return fUniqueID; }
     virtual bool isOpaque() const { return false; }
     virtual size_t getSize() const = 0;
 
     /**
      *  Draw this SpecialImage into the canvas.
      */
     void draw(SkCanvas*, SkScalar x, SkScalar y, const SkPaint*) const;
 
     static SkSpecialImage* NewFromImage(const SkIRect& subset, const SkImage*);
@@ skipping 17 matching lines @@
     SkImageFilter::Proxy* internal_getProxy();
 
 protected:
     SkSpecialImage(SkImageFilter::Proxy* proxy, const SkIRect& subset, uint32_t uniqueID)
         : fSubset(subset)
         , fUniqueID(kNeedNewImageUniqueID_SpecialImage == uniqueID ? SkNextID::ImageID()
                                                                    : uniqueID)
         , fProxy(proxy) {
     }
 
-    // The following 3 are for testing and shouldn't be used. (see skbug.com/4965)
+    // The following 2 are for testing and shouldn't be used.
     friend class TestingSpecialImageAccess;
     friend class TestingSpecialSurfaceAccess;
-    const SkIRect& subset() const { return fSubset; }
 
     /**
      *  If the SpecialImage is backed by cpu pixels, return the const address
      *  of those pixels and, if not null, return the ImageInfo and rowBytes.
      *  The returned address is only valid while the image object is in scope.
      *
      *  The returned ImageInfo represents the backing memory. Use 'subset'
      *  to get the active portion's dimensions.
      *
      *  On failure, return false and ignore the pixmap parameter.
@@ skipping 14 matching lines @@
     const uint32_t  fUniqueID;
 
     // TODO: remove this ASAP (see skbug.com/4965)
     SkImageFilter::Proxy* fProxy;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif