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/RawDecoder.cxx b/common/rfb/RawDecoder.cxx
index e349bda..786f154 100644
--- a/common/rfb/RawDecoder.cxx
+++ b/common/rfb/RawDecoder.cxx
@@ -25,7 +25,7 @@
 
 using namespace rfb;
 
-RawDecoder::RawDecoder()
+RawDecoder::RawDecoder() : Decoder(DecoderPlain)
 {
 }