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

include/core/SkCanvas.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCanvas_DEFINED
 #define SkCanvas_DEFINED
 
@@ skipping 1339 matching lines @@
     // Clip rectangle bounds. Called internally by saveLayer.
     // returns false if the entire rectangle is entirely clipped out
     // If non-NULL, The imageFilter parameter will be used to expand the clip
     // and offscreen bounds for any margin required by the filter DAG.
     bool clipRectBounds(const SkRect* bounds, SaveFlags flags,
                         SkIRect* intersection,
                         const SkImageFilter* imageFilter = NULL);
 
     // 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;
 
     SkAutoTUnref<SkClipStack> fClipStack;
     SkDeque     fMCStack;
     // points to top of stack
     MCRec*      fMCRec;
     // the first N recs that can fit here mean we won't call malloc
     enum {
@@ skipping 75 matching lines @@
     void internalRestore();
     static void DrawRect(const SkDraw& draw, const SkPaint& paint,
                          const SkRect& r, SkScalar textSize);
     static void DrawTextDecorations(const SkDraw& draw, const SkPaint& paint,
                                     const char text[], size_t byteLength,
                                     SkScalar x, SkScalar y);
 
     // 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).
      */
     mutable SkRect fCachedLocalClipBounds;
     mutable bool   fCachedLocalClipBoundsDirty;
     bool fAllowSoftClip;
     bool fAllowSimplifyClip;
     bool fConservativeRasterClip;
 
     const SkRect& getLocalClipBounds() const {
@@ skipping 119 matching lines @@
 
 class SkCanvasClipVisitor {
 public:
     virtual ~SkCanvasClipVisitor();
     virtual void clipRect(const SkRect&, SkRegion::Op, bool antialias) = 0;
     virtual void clipRRect(const SkRRect&, SkRegion::Op, bool antialias) = 0;
     virtual void clipPath(const SkPath&, SkRegion::Op, bool antialias) = 0;
 };
 
 #endif