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.
diff --git a/common/rfb/TightDecoder.cxx b/common/rfb/TightDecoder.cxx
index 86bb006..3a1254a 100644
--- a/common/rfb/TightDecoder.cxx
+++ b/common/rfb/TightDecoder.cxx
@@ -173,11 +173,6 @@
   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 @@
     int stride;
     rdr::U8 *buf;
 
+    JpegDecompressor jd;
+
     assert(buflen >= 4);
 
     memcpy(&len, bufptr, 4);