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

include/gpu/GrRenderTarget.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrRenderTarget_DEFINED
 #define GrRenderTarget_DEFINED
 
@@ skipping 103 matching lines @@
     const SkIRect& getResolveRect() const { return fResolveRect; }
 
     /**
      * If the render target is multisampled this will perform a multisample
      * resolve. Any pending draws to the target are first flushed. This only
      * applies to render targets that are associated with GrTextures. After the
      * function returns the GrTexture will contain the resolved pixels.
      */
     void resolve();
 
+    /**
+     * Provide a performance hint that the render target's contents are allowed
+     * to become undefined.
+     */
+    void discard();
+
     // a MSAA RT may require explicit resolving , it may auto-resolve (e.g. FBO
     // 0 in GL), or be unresolvable because the client didn't give us the
     // resolve destination.
     enum ResolveType {
         kCanResolve_ResolveType,
         kAutoResolves_ResolveType,
         kCantResolve_ResolveType,
     };
     virtual ResolveType getResolveType() const = 0;
 
@@ skipping 28 matching lines @@
 
     GrStencilBuffer*  fStencilBuffer;
     GrTexture*        fTexture; // not ref'ed
 
     SkIRect           fResolveRect;
 
     typedef GrSurface INHERITED;
 };
 
 #endif