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

src/image/SkSurface.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSurface_Base.h"
 #include "SkImagePriv.h"
 #include "SkCanvas.h"
@@ skipping 68 matching lines @@
 }
 
 void SkSurface_Base::onDraw(SkCanvas* canvas, SkScalar x, SkScalar y, const SkPaint* paint) {
     SkImage* image = this->newImageSnapshot(kYes_Budgeted);
     if (image) {
         canvas->drawImage(image, x, y, paint);
         image->unref();
     }
 }
 
+bool SkSurface_Base::outstandingImageSnapshot() const {
+    return fCachedImage && !fCachedImage->unique();
+}
+
 void SkSurface_Base::aboutToDraw(ContentChangeMode mode) {
     this->dirtyGenerationID();
 
     SkASSERT(!fCachedCanvas || fCachedCanvas->getSurfaceBase() == this);
 
     if (fCachedImage) {
         // the surface may need to fork its backend, if its sharing it with
         // the cached image. Note: we only call if there is an outstanding owner
         // on the image (besides us).
         if (!fCachedImage->unique()) {
@@ skipping 101 matching lines @@
                                              const SkSurfaceProps*) {
     return NULL;
 }
 
 SkSurface* SkSurface::NewFromBackendRenderTarget(GrContext*, const GrBackendRenderTargetDesc&,
                                                  const SkSurfaceProps*) {
     return NULL;
 }
 
 #endif