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

src/core/SkBlitter.h

Index: src/core/SkBlitter.h
diff --git a/src/core/SkBlitter.h b/src/core/SkBlitter.h
index d19a34badcd4d9ecbb78cd6f22f074f2ff0927d5..fbe18ebc269dd19fefcf2f998259dea9d951fda6 100644
--- a/src/core/SkBlitter.h
+++ b/src/core/SkBlitter.h
@@ -61,6 +61,12 @@ public:
      */
     virtual bool isNullBlitter() const;
 
+    /**
+     *  Special method for SkShaderBlitter. On all other classes this is a no-op.
+     */
+    virtual bool resetShaderContext(const SkBitmap& device, const SkPaint& paint,
+                                    const SkMatrix& matrix);
+
     ///@name non-virtual helpers
     void blitMaskRegion(const SkMask& mask, const SkRegion& clip);
     void blitRectRegion(const SkIRect& rect, const SkRegion& clip);