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

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 25 matching lines @@
     SkImage* image = this->newImageSnapshot();
     if (image) {
         image->draw(canvas, x, y, paint);
         image->unref();
     }
 }
 
 void SkSurface_Base::aboutToDraw(ContentChangeMode mode) {
     this->dirtyGenerationID();
 
-    if (NULL != fCachedCanvas) {
-        SkASSERT(fCachedCanvas->getSurfaceBase() == this || \
-                 NULL == fCachedCanvas->getSurfaceBase());
-        fCachedCanvas->setSurfaceBase(NULL);
-    }
+    SkASSERT(!fCachedCanvas || fCachedCanvas->getSurfaceBase() == this);
 
     if (NULL != 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()) {
             this->onCopyOnWrite(mode);
         }
 
         // regardless of copy-on-write, we must drop our cached image now, so
         // that the next request will get our new contents.
         fCachedImage->unref();
         fCachedImage = NULL;
+    } else if (kDiscard_ContentChangeMode == mode) {
+        this->onDiscard();
     }
 }
 
 uint32_t SkSurface_Base::newGenerationID() {
-    this->installIntoCanvasForDirtyNotification();
-
+    SkASSERT(!fCachedCanvas || fCachedCanvas->getSurfaceBase() == this);
     static int32_t gID;
     return sk_atomic_inc(&gID) + 1;
 }
 
 static SkSurface_Base* asSB(SkSurface* surface) {
     return static_cast<SkSurface_Base*>(surface);
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
@@ skipping 38 matching lines @@
 }
 
 void SkSurface::draw(SkCanvas* canvas, SkScalar x, SkScalar y,
                      const SkPaint* paint) {
     return asSB(this)->onDraw(canvas, x, y, paint);
 }
 
 const void* SkSurface::peekPixels(SkImageInfo* info, size_t* rowBytes) {
     return this->getCanvas()->peekPixels(info, rowBytes);
 }