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/Encoder.h b/common/rfb/Encoder.h
index 66a10d2..1e57fda 100644
--- a/common/rfb/Encoder.h
+++ b/common/rfb/Encoder.h
@@ -42,7 +42,7 @@
   class Encoder {
   public:
     Encoder(SConnection* conn, int encoding,
-            enum EncoderFlags flags, unsigned int maxPaletteSize);
+            enum EncoderFlags flags, unsigned int maxPaletteSize=-1);
     virtual ~Encoder();
 
     // isSupported() should return a boolean indicating if this encoder