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/Decoder.cxx b/common/rfb/Decoder.cxx
index a1f4438..f9bbffc 100644
--- a/common/rfb/Decoder.cxx
+++ b/common/rfb/Decoder.cxx
@@ -28,7 +28,7 @@
 
 using namespace rfb;
 
-Decoder::Decoder()
+Decoder::Decoder(enum DecoderFlags flags) : flags(flags)
 {
 }