Add flags member to decoders

Allows us to add attributes later that affect of the decoder
manager deals with the decoders.
diff --git a/common/rfb/TightDecoder.cxx b/common/rfb/TightDecoder.cxx
index 1bfa7a9..24236db 100644
--- a/common/rfb/TightDecoder.cxx
+++ b/common/rfb/TightDecoder.cxx
@@ -46,7 +46,7 @@
 #include <rfb/tightDecode.h>
 #undef BPP
 
-TightDecoder::TightDecoder()
+TightDecoder::TightDecoder() : Decoder(DecoderPlain)
 {
 }