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/TightJPEGEncoder.cxx b/common/rfb/TightJPEGEncoder.cxx
index 385207f..5d39b62 100644
--- a/common/rfb/TightJPEGEncoder.cxx
+++ b/common/rfb/TightJPEGEncoder.cxx
@@ -64,7 +64,7 @@
 
 
 TightJPEGEncoder::TightJPEGEncoder(SConnection* conn) :
-  Encoder(conn, encodingTight, (EncoderFlags)(EncoderUseNativePF | EncoderLossy), -1),
+  Encoder(conn, encodingTight, (EncoderFlags)(EncoderUseNativePF | EncoderLossy)),
   qualityLevel(-1), fineQuality(-1), fineSubsampling(subsampleUndefined)
 {
 }