clear stencil buffer using special purpose FBO

src/gpu/gl/GrGLGpu.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 GrGLGpu_DEFINED
 #define GrGLGpu_DEFINED
 
@@ skipping 294 matching lines @@
 
     enum TriState {
         kNo_TriState,
         kYes_TriState,
         kUnknown_TriState
     };
 
     GrGLuint                    fTempSrcFBOID;
     GrGLuint                    fTempDstFBOID;
 
+    GrGLuint                    fStencilClearFBOID;
+
     // last scissor / viewport scissor state seen by the GL.
     struct {
         TriState    fEnabled;
         GrGLIRect   fRect;
         void invalidate() {
             fEnabled = kUnknown_TriState;
             fRect.invalidate();
         }
     } fHWScissorSettings;
 
@@ skipping 138 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;
     friend class GrGLPathRendering; // For accessing setTextureUnit.
 };
 
 #endif