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/RREDecoder.cxx b/common/rfb/RREDecoder.cxx
index 2dd47cc..218c9b0 100644
--- a/common/rfb/RREDecoder.cxx
+++ b/common/rfb/RREDecoder.cxx
@@ -36,7 +36,7 @@
 #include <rfb/rreDecode.h>
 #undef BPP
 
-RREDecoder::RREDecoder()
+RREDecoder::RREDecoder() : Decoder(DecoderPlain)
 {
 }