have canvas send discard instead of retain if the draw would overwrite everything

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index ca6818518d74078401d0d73a1b3cfdc61d842010..d454d082d6477c779f281903c71602ab7ec5ac4a 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -1357,7 +1357,8 @@ protected:
 
     // notify our surface (if we have one) that we are about to draw, so it
     // can perform copy-on-write or invalidate any cached images
-    void predrawNotify();
+    void predrawNotify(bool completeOverwrite = false);
+    void predrawNotify(const SkRect*, const SkPaint*);
 
 private:
     class MCRec;
@@ -1453,6 +1454,9 @@ private:
     // only for canvasutils
     const SkRegion& internal_private_getTotalClip() const;
 
+    bool willOverwriteAllPixels(const SkRect*, const SkPaint*) const;
+
+
     /*  These maintain a cache of the clip bounds in local coordinates,
         (converted to 2s-compliment if floats are slow).
      */