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

include/gpu/GrContext.h

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrContext_DEFINED
 #define GrContext_DEFINED
 
@@ skipping 635 matching lines @@
      * Resolves a render target that has MSAA. The intermediate MSAA buffer is
      * down-sampled to the associated GrTexture (accessible via
      * GrRenderTarget::asTexture()). Any pending draws to the render target will
      * be executed before the resolve.
      *
      * This is only necessary when a client wants to access the object directly
      * using the backend API directly. GrContext will detect when it must
      * perform a resolve to a GrTexture used as the source of a draw or before
      * reading pixels back from a GrTexture or GrRenderTarget.
      */
-    void resolveRenderTarget(GrRenderTarget* target);
+    void resolveRenderTarget(GrRenderTarget*);
+
+    /**
+     * Provides a perfomance hint that the render target's contents are allowed
+     * to become undefined.
+     */
+    void discardRenderTarget(GrRenderTarget*);
 
 #ifdef SK_DEVELOPER
     void dumpFontCache() const;
 #endif
 
     ///////////////////////////////////////////////////////////////////////////
     // Helpers
 
     class AutoRenderTarget : public ::SkNoncopyable {
     public:
@@ skipping 402 matching lines @@
     }
 
     GrTexture* texture() { return fTexture; }
 
 private:
     GrContext*                    fContext;
     GrTexture*                    fTexture;
 };
 
 #endif