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

src/gpu/gl/GrGpuGL.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 GrGpuGL_DEFINED
 #define GrGpuGL_DEFINED
 
@@ skipping 22 matching lines @@
 
     const GrGLContext& glContext() const { return fGLContext; }
 
     const GrGLInterface* glInterface() const { return fGLContext.interface(); }
     const GrGLContextInfo& ctxInfo() const { return fGLContext; }
     GrGLStandard glStandard() const { return fGLContext.standard(); }
     GrGLVersion glVersion() const { return fGLContext.version(); }
     GrGLSLGeneration glslGeneration() const { return fGLContext.glslGeneration(); }
     const GrGLCaps& glCaps() const { return *fGLContext.caps(); }
 
+    virtual void discard(GrRenderTarget*) SK_OVERRIDE;
+
     // Used by GrGLProgram and GrGLTexGenProgramEffects to configure OpenGL state.
     void bindTexture(int unitIdx, const GrTextureParams& params, GrGLTexture* texture);
     void setProjectionMatrix(const SkMatrix& matrix,
                              const SkISize& renderTargetSize,
                              GrSurfaceOrigin renderTargetOrigin);
     enum TexGenComponents {
         kS_TexGenComponents = 1,
         kST_TexGenComponents = 2,
         kSTR_TexGenComponents = 3
     };
@@ skipping 401 matching lines @@
     ///@}
 
     // we record what stencil format worked last time to hopefully exit early
     // from our loop that tries stencil formats and calls check fb status.
     int fLastSuccessfulStencilFmtIdx;
 
     typedef GrGpu INHERITED;
 };
 
 #endif