Add discard API to SkCanvas, plumb it to glDiscardFramebuffer()

src/image/SkSurface_Picture.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 "SkCanvas.h"
 #include "SkImagePriv.h"
 #include "SkPicture.h"
 
 /**
  *  What does it mean to ask for more than one canvas from a picture?
  *  How do we return an Image and then "continue" recording?
  */
 class SkSurface_Picture : public SkSurface_Base {
 public:
     SkSurface_Picture(int width, int height);
     virtual ~SkSurface_Picture();
 
     virtual SkCanvas* onNewCanvas() SK_OVERRIDE;
     virtual SkSurface* onNewSurface(const SkImageInfo&) SK_OVERRIDE;
     virtual SkImage* onNewImageSnapshot() SK_OVERRIDE;
     virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y,
                         const SkPaint*) SK_OVERRIDE;
     virtual void onCopyOnWrite(ContentChangeMode) SK_OVERRIDE;
+    virtual void onDiscard() SK_OVERRIDE;
 
 private:
     SkPicture*  fPicture;
 
     typedef SkSurface_Base INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 SkSurface_Picture::SkSurface_Picture(int width, int height) : INHERITED(width, height) {
@@ skipping 30 matching lines @@
 }
 
 void SkSurface_Picture::onDraw(SkCanvas* canvas, SkScalar x, SkScalar y,
                                const SkPaint* paint) {
     if (!fPicture) {
         return;
     }
     SkImagePrivDrawPicture(canvas, fPicture, x, y, paint);
 }
 
-void SkSurface_Picture::onCopyOnWrite(ContentChangeMode /*mode*/) {
+void SkSurface_Picture::onCopyOnWrite(ContentChangeMode mode) {
     // We always spawn a copy of the recording picture when we
     // are asked for a snapshot, so we never need to do anything here.
+    if (kDiscard_ContentChangeMode == mode) {
+        this->SkSurface_Picture::onDiscard();
+    }
+}
+
+void SkSurface_Picture::onDiscard() {
+    if (NULL != fPicture) {
+        fPicture->beginRecording(this->width(), this->height());
+    }
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 
 SkSurface* SkSurface::NewPicture(int width, int height) {
     if ((width | height) < 0) {
         return NULL;
     }
 
     return SkNEW_ARGS(SkSurface_Picture, (width, height));
 }