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.

common/rfb/Encoder.h

   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

common/rfb/HextileEncoder.cxx

 #undef BPP
 
 HextileEncoder::HextileEncoder(SConnection* conn) :
-  Encoder(conn, encodingHextile, EncoderPlain, -1)
+  Encoder(conn, encodingHextile, EncoderPlain)
 {
 }
 

common/rfb/RREEncoder.cxx

 #undef BPP
 
 RREEncoder::RREEncoder(SConnection* conn) :
-  Encoder(conn, encodingRRE, EncoderPlain, -1)
+  Encoder(conn, encodingRRE, EncoderPlain)
 {
 }
 

common/rfb/RawEncoder.cxx

 using namespace rfb;
 
 RawEncoder::RawEncoder(SConnection* conn) :
-  Encoder(conn, encodingRaw, EncoderPlain, -1)
+  Encoder(conn, encodingRaw, EncoderPlain)
 {
 }
 

common/rfb/TightJPEGEncoder.cxx

 
 
 TightJPEGEncoder::TightJPEGEncoder(SConnection* conn) :
-  Encoder(conn, encodingTight, (EncoderFlags)(EncoderUseNativePF | EncoderLossy), -1),
+  Encoder(conn, encodingTight, (EncoderFlags)(EncoderUseNativePF | EncoderLossy)),
   qualityLevel(-1), fineQuality(-1), fineSubsampling(subsampleUndefined)
 {
 }