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

src/gpu/GrInOrderDrawBuffer.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 GrInOrderDrawBuffer_DEFINED
 #define GrInOrderDrawBuffer_DEFINED
 
@@ skipping 54 matching lines @@
 
     // tracking for draws
     virtual DrawToken getCurrentDrawToken() { return DrawToken(this, fDrawID); }
 
     // overrides from GrDrawTarget
     virtual bool geometryHints(int* vertexCount,
                                int* indexCount) const SK_OVERRIDE;
     virtual void clear(const SkIRect* rect,
                        GrColor color,
                        bool canIgnoreRect,
-                       GrRenderTarget* renderTarget = NULL) SK_OVERRIDE;
+                       GrRenderTarget* renderTarget) SK_OVERRIDE;
+
+    virtual void discard(GrRenderTarget*) SK_OVERRIDE;
 
     virtual void initCopySurfaceDstDesc(const GrSurface* src, GrTextureDesc* desc) SK_OVERRIDE;
 
 protected:
     virtual void clipWillBeSet(const GrClipData* newClip) SK_OVERRIDE;
 
 private:
     enum Cmd {
         kDraw_Cmd           = 1,
         kStencilPath_Cmd    = 2,
@@ skipping 32 matching lines @@
         ~DrawPaths();
 
         size_t fPathCount;
         const GrPath** fPaths;
         SkMatrix* fTransforms;
         SkPath::FillType fFill;
         SkStrokeRec::Style fStroke;
         GrDeviceCoordTexture fDstCopy;
     };
 
+    // This is also used to record a discard by setting the color to GrColor_ILLEGAL
     struct Clear : public ::SkNoncopyable {
         Clear() : fRenderTarget(NULL) {}
         ~Clear() { SkSafeUnref(fRenderTarget); }
 
         SkIRect         fRect;
         GrColor         fColor;
         bool            fCanIgnoreRect;
         GrRenderTarget* fRenderTarget;
     };
 
@@ skipping 127 matching lines @@
 
     void addToCmdBuffer(uint8_t cmd);
 
     bool                            fFlushing;
     uint32_t                        fDrawID;
 
     typedef GrDrawTarget INHERITED;
 };
 
 #endif