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

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 29 matching lines @@
      *
      *  image = this->newImageSnapshot();
      *  if (image) {
      *      image->draw(canvas, ...);
      *      image->unref();
      *  }
      */
     virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y, const SkPaint*);
 
     /**
+     * Called as a performance hint when the Surface is allowed to make it's contents
+     * undefined.
+     */
+    virtual void onDiscard() {}
+
+    /**
      *  If the surface is about to change, we call this so that our subclass
      *  can optionally fork their backend (copy-on-write) in case it was
      *  being shared with the cachedImage.
      */
     virtual void onCopyOnWrite(ContentChangeMode) = 0;
 
     inline SkCanvas* getCachedCanvas();
     inline SkImage* getCachedImage();
 
     // called by SkSurface to compute a new genID
     uint32_t newGenerationID();
 
 private:
     SkCanvas*   fCachedCanvas;
     SkImage*    fCachedImage;
 
     void aboutToDraw(ContentChangeMode mode);
     friend class SkCanvas;
     friend class SkSurface;
 
-    inline void installIntoCanvasForDirtyNotification();
-
     typedef SkSurface INHERITED;
 };
 
 SkCanvas* SkSurface_Base::getCachedCanvas() {
     if (NULL == fCachedCanvas) {
         fCachedCanvas = this->onNewCanvas();
-        this->installIntoCanvasForDirtyNotification();
+        if (NULL != fCachedCanvas) {
+            fCachedCanvas->setSurfaceBase(this);
+        }
     }
     return fCachedCanvas;
 }
 
 SkImage* SkSurface_Base::getCachedImage() {
     if (NULL == fCachedImage) {
         fCachedImage = this->onNewImageSnapshot();
-        this->installIntoCanvasForDirtyNotification();
+        SkASSERT(!fCachedCanvas || fCachedCanvas->getSurfaceBase() == this);
     }
     return fCachedImage;
 }
 
-void SkSurface_Base::installIntoCanvasForDirtyNotification() {
-    if (NULL != fCachedCanvas) {
-        fCachedCanvas->setSurfaceBase(this);
-    }
-}
-
 #endif