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

src/image/SkSurface_Gpu.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"
 #include "SkGpuDevice.h"
 
 class SkSurface_Gpu : public SkSurface_Base {
 public:
     SK_DECLARE_INST_COUNT(SkSurface_Gpu)
 
     SkSurface_Gpu(GrRenderTarget*, bool cached);
     virtual ~SkSurface_Gpu();
 
     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:
     SkGpuDevice* fDevice;
 
     typedef SkSurface_Base INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 SkSurface_Gpu::SkSurface_Gpu(GrRenderTarget* renderTarget, bool cached)
@@ skipping 43 matching lines @@
             fDevice->createCompatibleDevice(fDevice->imageInfo()));
         SkAutoTUnref<SkGpuDevice> aurd(newDevice);
         if (kRetain_ContentChangeMode == mode) {
             fDevice->context()->copyTexture(rt->asTexture(), newDevice->accessRenderTarget());
         }
         SkASSERT(NULL != this->getCachedCanvas());
         SkASSERT(this->getCachedCanvas()->getDevice() == fDevice);
 
         this->getCachedCanvas()->setRootDevice(newDevice);
         SkRefCnt_SafeAssign(fDevice, newDevice);
+    } else if (kDiscard_ContentChangeMode == mode) {
+        this->SkSurface_Gpu::onDiscard();
     }
 }
 
+void SkSurface_Gpu::onDiscard() {
+    fDevice->accessRenderTarget()->discard();
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 SkSurface* SkSurface::NewRenderTargetDirect(GrRenderTarget* target) {
     if (NULL == target) {
         return NULL;
     }
     return SkNEW_ARGS(SkSurface_Gpu, (target, false));
 }
 
 SkSurface* SkSurface::NewRenderTarget(GrContext* ctx, const SkImageInfo& info, int sampleCount) {
@@ skipping 33 matching lines @@
     desc.fSampleCnt = sampleCount;
 
     SkAutoTUnref<GrTexture> tex(ctx->lockAndRefScratchTexture(desc, GrContext::kExact_ScratchTexMatch));
 
     if (NULL == tex) {
         return NULL;
     }
 
     return SkNEW_ARGS(SkSurface_Gpu, (tex->asRenderTarget(), true));
 }