Add a SkRWBuffer reserve mechanism

include/core/SkRWBuffer.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkRWBuffer_DEFINED
 #define SkRWBuffer_DEFINED
 
@@ skipping 61 matching lines @@
  *  The growth is done such that at any time in the writer's thread, an RBuffer or StreamAsset
  *  can be snapped off (and safely passed to another thread). The RBuffer/StreamAsset snapshot
  *  can see the previously stored bytes, but will be unaware of any future writes.
  */
 class SK_API SkRWBuffer {
 public:
     SkRWBuffer(size_t initialCapacity = 0);
     ~SkRWBuffer();
 
     size_t size() const { return fTotalUsed; }
-    void append(const void* buffer, size_t length);
+
+    /**
+     *  Append |length| bytes from |buffer|.
+     *
+     *  If the caller knows in advance how much more data they are going to append, they can
+     *  pass a |reserve| hint (representing the number of upcoming bytes *in addition* to the
+     *  current append), to minimize the number of internal allocations.
+     */
+    void append(const void* buffer, size_t length, size_t reserve = 0);
 
     SkROBuffer* newRBufferSnapshot() const;
     SkStreamAsset* newStreamSnapshot() const;
 
 #ifdef SK_DEBUG
     void validate() const;
 #else
     void validate() const {}
 #endif
     
 private:
     SkBufferHead*   fHead;
     SkBufferBlock*  fTail;
     size_t          fTotalUsed;
 };
 
 #endif