Add write protection to OffsetPixelBuffer

No one should every try to write to this buffer. Enforce that by
throwing an exception if any one tries to get a writeable pointer
to the data.

common/rfb/EncodeManager.h

@@ -187,6 +187,9 @@ namespace rfb {
 
       void update(const PixelFormat& pf, int width, int height,
                   const rdr::U8* data_, int stride);
+
+    private:
+      virtual rdr::U8* getBufferRW(const Rect& r, int* stride);
     };
   };
 }