Add a SkRWBuffer reserve mechanism

include/core/SkRWBuffer.h

Index: include/core/SkRWBuffer.h
diff --git a/include/core/SkRWBuffer.h b/include/core/SkRWBuffer.h
index 9626d28df9643ee836ba6a1e285ce456ec7101f0..451933f353cae3a367c57f74e239c137794aa4f1 100644
--- a/include/core/SkRWBuffer.h
+++ b/include/core/SkRWBuffer.h
@@ -79,7 +79,15 @@ public:
     ~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;