Add transfer buffer support.

src/gpu/gl/GrGLBufferImpl.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLBufferImpl_DEFINED
 #define GrGLBufferImpl_DEFINED
 
 #include "SkTypes.h"
 #include "gl/GrGLTypes.h"
 
 class GrGLGpu;
 
 /**
  * This class serves as the implementation of GrGL*Buffer classes. It was written to avoid code
  * duplication in those classes.
  */
 class GrGLBufferImpl : SkNoncopyable {
 public:
+    enum Usage {
+        kStaticDraw_Usage = 0,
+        kDynamicDraw_Usage,
+        kStreamDraw_Usage,
+        kStreamRead_Usage,
+
+        kLast_Usage = kStreamRead_Usage
+    };
+    static const int kUsageCount = kLast_Usage + 1;
+
     struct Desc {
         GrGLuint    fID;            // set to 0 to indicate buffer is CPU-backed and not a VBO.
         size_t      fSizeInBytes;
-        bool        fDynamic;
+        Usage       fUsage;
     };
 
     GrGLBufferImpl(GrGLGpu*, const Desc&, GrGLenum bufferType);
     ~GrGLBufferImpl() {
         // either release or abandon should have been called by the owner of this object.
         SkASSERT(0 == fDesc.fID);
     }
 
     void abandon();
     void release(GrGLGpu* gpu);
@@ skipping 13 matching lines @@
     GrGLenum     fBufferType; // GL_ARRAY_BUFFER or GL_ELEMENT_ARRAY_BUFFER
     void*        fCPUData;
     void*        fMapPtr;
     size_t       fGLSizeInBytes;     // In certain cases we make the size of the GL buffer object
                                      // smaller or larger than the size in fDesc.
 
     typedef SkNoncopyable INHERITED;
 };
 
 #endif