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/HextileDecoder.cxx b/common/rfb/HextileDecoder.cxx
index d1412d4..eae0040 100644
--- a/common/rfb/HextileDecoder.cxx
+++ b/common/rfb/HextileDecoder.cxx
@@ -36,7 +36,7 @@
 #include <rfb/hextileDecode.h>
 #undef BPP
 
-HextileDecoder::HextileDecoder()
+HextileDecoder::HextileDecoder() : Decoder(DecoderPlain)
 {
 }