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/CopyRectDecoder.cxx b/common/rfb/CopyRectDecoder.cxx
index 52de879..32ed1b3 100644
--- a/common/rfb/CopyRectDecoder.cxx
+++ b/common/rfb/CopyRectDecoder.cxx
@@ -22,7 +22,7 @@
 
 using namespace rfb;
 
-CopyRectDecoder::CopyRectDecoder()
+CopyRectDecoder::CopyRectDecoder() : Decoder(DecoderPlain)
 {
 }