Make it so that drivenengine never changes (only appends) output bytes.

luprex/core/cpp/streambuffer.hpp

@@ -347,7 +347,7 @@ public:
     // Read the entire contents of the buffer as a string.
     //
     std::string read_entire_contents();
-    
+
     // Overwrite values previously written to the buffer.
     //
     // See the comment at the top of this file for an explanation.
@@ -384,6 +384,9 @@ public:
     // Copy the entire contents of this streambuffer into another one.
     void copy_into(StreamBuffer *sb);
 
+    // Transfer the entire contents of this streambuffer into another one.
+    void transfer_into(StreamBuffer *sb);
+
     // Compare the contents of this streambuffer to another one.
     bool contents_equal(const StreamBuffer *sb) const;