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/RawEncoder.cxx b/common/rfb/RawEncoder.cxx
index 4090427..b12cf06 100644
--- a/common/rfb/RawEncoder.cxx
+++ b/common/rfb/RawEncoder.cxx
@@ -25,7 +25,7 @@
 using namespace rfb;
 
 RawEncoder::RawEncoder(SConnection* conn) :
-  Encoder(conn, encodingRaw, EncoderPlain, -1)
+  Encoder(conn, encodingRaw, EncoderPlain)
 {
 }