symbol->png will also accept fpng as a stream

thanks for that nice lib! I needed a qr-code dynamically created for use with hunchentoot - so I needed to write the png-image to a stream, changes attached.

example - serve a dynamic qr-code with hunchentoot:

(ql:quickload :hunchentoot)
(hunchentoot:start (make-instance 'hunchentoot:easy-acceptor :port 8080))
(hunchentoot:define-easy-handler (teste :uri "/test")
    ()
  (setf (hunchentoot:content-type*) "image/png")
  (let ((out (hunchentoot:send-headers)))
    (cl-qrencode:encode-png "my qr-code" :fpath out)))

point your browser to http://localhost:8080/test