Add a SkRWBuffer reserve mechanism

src/core/SkRWBuffer.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAtomics.h"
 #include "SkRWBuffer.h"
 #include "SkStream.h"
@@ skipping 176 matching lines @@
     this->validate();
     if (fHead) {
         fHead->unref();
     }
 }
 
 // It is important that we always completely fill the current block before spilling over to the
 // next, since our reader will be using fCapacity (min'd against its total available) to know how
 // many bytes to read from a given block.
 //
-void SkRWBuffer::append(const void* src, size_t length) {
+void SkRWBuffer::append(const void* src, size_t length, size_t reserve) {
     this->validate();
     if (0 == length) {
         return;
     }
 
     fTotalUsed += length;
 
     if (nullptr == fHead) {
-        fHead = SkBufferHead::Alloc(length);
+        fHead = SkBufferHead::Alloc(length + reserve);
         fTail = &fHead->fBlock;
     }
 
     size_t written = fTail->append(src, length);
     SkASSERT(written <= length);
     src = (const char*)src + written;
     length -= written;
 
     if (length) {
-        SkBufferBlock* block = SkBufferBlock::Alloc(length);
+        SkBufferBlock* block = SkBufferBlock::Alloc(length + reserve);
         fTail->fNext = block;
         fTail = block;
         written = fTail->append(src, length);
         SkASSERT(written == length);
     }
     this->validate();
 }
 
 #ifdef SK_DEBUG
 void SkRWBuffer::validate() const {
@@ skipping 119 matching lines @@
     const SkROBuffer*   fBuffer;
     SkROBuffer::Iter    fIter;
     size_t              fLocalOffset;
     size_t              fGlobalOffset;
 };
 
 SkStreamAsset* SkRWBuffer::newStreamSnapshot() const {
     SkAutoTUnref<SkROBuffer> buffer(this->newRBufferSnapshot());
     return new SkROBufferStreamAsset(buffer);
 }