Don't force encoders to specify palette size

Most encoders don't have a palette mode, so don't force them to
specify a dummy value to the Encoder base class constructor.
diff --git a/common/rfb/RREEncoder.cxx b/common/rfb/RREEncoder.cxx
index 60a0663..7287e7e 100644
--- a/common/rfb/RREEncoder.cxx
+++ b/common/rfb/RREEncoder.cxx
@@ -37,7 +37,7 @@
 #undef BPP
 
 RREEncoder::RREEncoder(SConnection* conn) :
-  Encoder(conn, encodingRRE, EncoderPlain, -1)
+  Encoder(conn, encodingRRE, EncoderPlain)
 {
 }