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

src/gpu/GrDrawTarget.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 GrDrawTarget_DEFINED
 #define GrDrawTarget_DEFINED
 
@@ skipping 420 matching lines @@
      * clip and all other draw state (blend mode, stages, etc). Clears the
      * whole thing if rect is NULL, otherwise just the rect. If canIgnoreRect
      * is set then the entire render target can be optionally cleared.
      */
     virtual void clear(const SkIRect* rect,
                        GrColor color,
                        bool canIgnoreRect,
                        GrRenderTarget* renderTarget = NULL) = 0;
 
     /**
+     * Discards the contents render target. NULL indicates that the current render target should
+     * be discarded.
+     **/
+    virtual void discard(GrRenderTarget* = NULL) = 0;
+
+    /**
      * Called at start and end of gpu trace marking
      * GR_CREATE_GPU_TRACE_MARKER(marker_str, target) will automatically call these at the start
      * and end of a code block respectively
      */
     void addGpuTraceMarker(GrGpuTraceMarker* marker);
     void removeGpuTraceMarker(GrGpuTraceMarker* marker);
 
     /**
      * Copies a pixel rectangle from one surface to another. This call may finalize
      * reserved vertex/index data (as though a draw call was made). The src pixels
@@ skipping 483 matching lines @@
     // The context owns us, not vice-versa, so this ptr is not ref'ed by DrawTarget.
     GrContext*                                                      fContext;
     // To keep track that we always have at least as many debug marker adds as removes
     int                                                             fGpuTraceMarkerCount;
     GrTraceMarkerSet                                                fActiveTraceMarkers;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif