Allow parallel decoding of Tight JPEG rects

The cost is some more load on malloc(), but it should be small
compared to the gains of spreading the CPU heavy JPEG decoding
over multiple CPUs.

common/rfb/TightDecoder.cxx

   if (((comp_ctl_a & 0x0f) & (comp_ctl_b & 0x0f)) != 0)
     return true;
 
-  // We have a shared JpegDecompressor, so one at a time
-  if (((comp_ctl_a >> 4) == tightJpeg) &&
-      ((comp_ctl_b >> 4) == tightJpeg))
-    return true;
-
   return false;
 }
 
     int stride;
     rdr::U8 *buf;
 
+    JpegDecompressor jd;
+
     assert(buflen >= 4);
 
     memcpy(&len, bufptr, 4);

common/rfb/TightDecoder.h

 
   private:
     rdr::ZlibInStream zis[4];
-    JpegDecompressor jd;
   };
 }