Simplify stream availability handling

Just have a simply number of bytes argument to avoid a lot of
complexity.

common/rfb/CMsgWriter.cxx

@@ -57,7 +57,7 @@ void CMsgWriter::writeSetPixelFormat(const PixelFormat& pf)
 void CMsgWriter::writeSetEncodings(int nEncodings, rdr::U32* encodings)
 {
   startMsg(msgTypeSetEncodings);
-  os->skip(1);
+  os->pad(1);
   os->writeU16(nEncodings);
   for (int i = 0; i < nEncodings; i++)
     os->writeU32(encodings[i]);

common/rfb/JpegCompressor.cxx

@@ -95,7 +95,7 @@ JpegEmptyOutputBuffer(j_compress_ptr cinfo)
   JpegCompressor *jc = dest->instance;
 
   jc->setptr(jc->getend());
-  jc->overrun(jc->getend() - jc->getstart(), 1);
+  jc->overrun(jc->getend() - jc->getstart());
   dest->pub.next_output_byte = jc->getptr();
   dest->pub.free_in_buffer = jc->avail();
 

common/rfb/JpegCompressor.h

@@ -49,8 +49,8 @@ namespace rfb {
 
     inline rdr::U8* getstart() { return start; }
 
-    virtual inline size_t overrun(size_t itemSize, size_t nItems) {
-      return MemOutStream::overrun(itemSize, nItems);
+    inline virtual void overrun(int needed) {
+      return MemOutStream::overrun(needed);
     }
 
   private: