Avoid re-rendering stencil clip for every draw with reducable clip stack

include/core/SkClipStack.h

Index: include/core/SkClipStack.h
diff --git a/include/core/SkClipStack.h b/include/core/SkClipStack.h
index 145552dc13aca12b5cc1fd19404896379eb0f6b2..0d6cfb296fc578e54ece0f8b7181d814d43cafd2 100644
--- a/include/core/SkClipStack.h
+++ b/include/core/SkClipStack.h
@@ -109,7 +109,7 @@ public:
             stack not to the element itself. That is the same clip path in different stacks will
             have a different ID since the elements produce different clip result in the context of
             their stacks. */
-        int32_t getGenID() const { return fGenID; }
+        int32_t getGenID() const { SkASSERT(kInvalidGenID != fGenID); return fGenID; }
 
         /**
          * Gets the bounds of the clip element, either the rect or path bounds. (Whether the shape
@@ -316,25 +316,12 @@ public:
     bool isWideOpen() const;
 
     /**
-     * Add a callback function that will be called whenever a clip state
-     * is no longer viable. This will occur whenever restore
-     * is called or when a clipDevRect or clipDevPath call updates the
-     * clip within an existing save/restore state. Each clip state is
-     * represented by a unique generation ID.
-     */
-    typedef void (*PFPurgeClipCB)(int genID, void* data);
-    void addPurgeClipCallback(PFPurgeClipCB callback, void* data) const;
-
-    /**
-     * Remove a callback added earlier via addPurgeClipCallback
-     */
-    void removePurgeClipCallback(PFPurgeClipCB callback, void* data) const;
-
-    /**
      * The generation ID has three reserved values to indicate special
      * (potentially ignorable) cases
      */
-    static const int32_t kInvalidGenID = 0;
+    static const int32_t kInvalidGenID = 0;     //!< Invalid id that is never returned by
+                                                //!< SkClipStack. Useful when caching clips
+                                                //!< based on GenID.
     static const int32_t kEmptyGenID = 1;       // no pixels writeable
     static const int32_t kWideOpenGenID = 2;    // all pixels writeable
 
@@ -440,29 +427,12 @@ private:
     // invalid ID.
     static int32_t     gGenID;
 
-    struct ClipCallbackData {
-        PFPurgeClipCB   fCallback;
-        void*           fData;
-
-        friend bool operator==(const ClipCallbackData& a,
-                               const ClipCallbackData& b) {
-            return a.fCallback == b.fCallback && a.fData == b.fData;
-        }
-    };
-
-    mutable SkTDArray<ClipCallbackData> fCallbackData;
-
     /**
      * Restore the stack back to the specified save count.
      */
     void restoreTo(int saveCount);
 
     /**
-     * Invoke all the purge callbacks passing in element's generation ID.
-     */
-    void purgeClip(Element* element);
-
-    /**
      * Return the next unique generation ID.
      */
     static int32_t GetNextGenID();