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/HextileEncoder.h b/common/rfb/HextileEncoder.h
index 705cbc6..723f539 100644
--- a/common/rfb/HextileEncoder.h
+++ b/common/rfb/HextileEncoder.h
@@ -24,7 +24,7 @@
 
   class HextileEncoder : public Encoder {
   public:
-    HextileEncoder(SMsgWriter* writer);
+    HextileEncoder(SConnection* conn);
     virtual ~HextileEncoder();
     virtual void writeRect(const Rect& r, TransImageGetter* ig);
   };