Encoders/decoders should track the connection object

The connection object is a much more appropriate object for the
decoders and encoders to keep track of. Besides the streams, it also
contains state like connection parameters.
diff --git a/common/rfb/RREDecoder.h b/common/rfb/RREDecoder.h
index bee634f..7b6cc45 100644
--- a/common/rfb/RREDecoder.h
+++ b/common/rfb/RREDecoder.h
@@ -24,7 +24,7 @@
 
   class RREDecoder : public Decoder {
   public:
-    RREDecoder(CMsgReader* reader);
+    RREDecoder(CConnection* conn);
     virtual ~RREDecoder();
     virtual void readRect(const Rect& r, CMsgHandler* handler);
   };