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

@@ -173,11 +173,6 @@ bool TightDecoder::doRectsConflict(const Rect& rectA,
   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;
 }
 
@@ -229,6 +224,8 @@ void TightDecoder::decodeRect(const Rect& r, const void* buffer,
     int stride;
     rdr::U8 *buf;
 
+    JpegDecompressor jd;
+
     assert(buflen >= 4);
 
     memcpy(&len, bufptr, 4);

common/rfb/TightDecoder.h

@@ -67,7 +67,6 @@ namespace rfb {
 
   private:
     rdr::ZlibInStream zis[4];
-    JpegDecompressor jd;
   };
 }