in decode-stream, use descriptor-byte-reader, if supplied
in decode-stream, use descriptor-byte-reader, if supplied

 * previously, we would always make a new dsecriptor, even if we
   passed one in via the descriptor kw arg. Now we use that, if
   supplied.

   BTW, all of this code is kind of a mess and there's a major OOAO
   violation between decode-stream and
   decode-stream-height-width-ncomp (which doesn't have this
   fix). Perhaps we should clean this all up a bit.

varjagg/cl-jpeg