Extract most of the mutable state of SkShader into a separate Context object.

include/core/SkComposeShader.h

Index: include/core/SkComposeShader.h
diff --git a/include/core/SkComposeShader.h b/include/core/SkComposeShader.h
index 1fefd1369a1c3eec87146f43abeacd02b9aa0c54..369955036541509c1bb200eddc36ab321255cddc 100644
--- a/include/core/SkComposeShader.h
+++ b/include/core/SkComposeShader.h
@@ -34,10 +34,29 @@ public:
     SkComposeShader(SkShader* sA, SkShader* sB, SkXfermode* mode = NULL);
     virtual ~SkComposeShader();
 
-    virtual bool setContext(const SkBitmap&, const SkPaint&,
-                            const SkMatrix&) SK_OVERRIDE;
-    virtual void endContext() SK_OVERRIDE;
-    virtual void shadeSpan(int x, int y, SkPMColor[], int count) SK_OVERRIDE;
+    virtual bool validContext(const SkBitmap&, const SkPaint&,
+                              const SkMatrix&) const SK_OVERRIDE;
+    virtual SkShader::Context* createContext(const SkBitmap&, const SkPaint&,
+                                             const SkMatrix&, void*) const SK_OVERRIDE;
+    virtual size_t contextSize() const SK_OVERRIDE;
+
+    class ComposeShaderContext : public SkShader::Context {
+    public:
+        // Takes ownership of contextA and contextB, and calls their destructors.
+        ComposeShaderContext(const SkComposeShader&, const SkBitmap&,
+                             const SkPaint&, const SkMatrix&,
+                             SkShader::Context* contextA, SkShader::Context* contextB);
+
+        virtual ~ComposeShaderContext();
+
+        virtual void shadeSpan(int x, int y, SkPMColor[], int count) SK_OVERRIDE;
+    private:
+        SkShader::Context* fShaderContextA;
+        SkShader::Context* fShaderContextB;
+
+        typedef SkShader::Context INHERITED;
+    };
+
 
     SK_DEVELOPER_TO_STRING()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkComposeShader)
@@ -52,6 +71,8 @@ private:
     SkShader*   fShaderB;
     SkXfermode* fMode;
 
+    friend class ComposeImpl;
+
     typedef SkShader INHERITED;
 };