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

src/image/SkSurface_Base.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSurface_Base_DEFINED
 #define SkSurface_Base_DEFINED
 
@@ skipping 61 matching lines @@
     inline SkImage* getCachedImage(Budgeted);
 
     // called by SkSurface to compute a new genID
     uint32_t newGenerationID();
 
 private:
     SkCanvas*   fCachedCanvas;
     SkImage*    fCachedImage;
 
     void aboutToDraw(ContentChangeMode mode);
+
+    // Returns true if there is an outstanding image-snapshot, indicating that a call to aboutToDraw
+    // would trigger a copy-on-write.
+    bool outstandingImageSnapshot() const;
+
     friend class SkCanvas;
     friend class SkSurface;
 
     typedef SkSurface INHERITED;
 };
 
 SkCanvas* SkSurface_Base::getCachedCanvas() {
     if (NULL == fCachedCanvas) {
         fCachedCanvas = this->onNewCanvas();
         if (fCachedCanvas) {
             fCachedCanvas->setSurfaceBase(this);
         }
     }
     return fCachedCanvas;
 }
 
 SkImage* SkSurface_Base::getCachedImage(Budgeted budgeted) {
     if (NULL == fCachedImage) {
         fCachedImage = this->onNewImageSnapshot(budgeted);
         SkASSERT(!fCachedCanvas || fCachedCanvas->getSurfaceBase() == this);
     }
     return fCachedImage;
 }
 
 #endif