clipping does not work with rectangle sets on CLX backend

[dkochmanski]

Steps to reproduce:

(defun tclip-df (pane w h)
  ;; pane is a sheetless medium
  (let* ((R1    (make-bounding-rectangle  1 1 (1- w)(1- h)))
         (outer (make-bounding-rectangle 10 10  50 50))
         (inner (make-bounding-rectangle 15 5 30 15))
         ;; everywhere but inner
         (clip  (region-difference r1 inner)) ; std rectangle-set
         )
    (multiple-value-call 'draw-rectangle*
      pane (bounding-rectangle* r1)
      :ink +red+ :filled nil :line-thickness 1)

    ;; clx can't handle region-set?
    ;; note in clx medium.lisp re: yx-banding needs fixing
    (draw-design pane clip :ink +light-pink+)
    (with-drawing-options (pane :clipping-region clip)
      #+ (or)
      (multiple-value-call 'draw-rectangle*
        pane (bounding-rectangle* inner)
        :ink +magenta+ :filled t)       ; should not be seen
      (multiple-value-call 'draw-rectangle*
        pane (bounding-rectangle* outer) ; no line in protected are
        :ink +blue+ :filled nil
        :line-thickness 1))))

(defun tclip ()
  (let* ((width 150)(height 150)
         (pane (open-window-stream
                :port (find-port)
                :left 1100 :top 120
                :max-width width :max-height height
                :scroll-bars nil :borders nil
                ;;:label "Test"
                )))
    (tclip-df pane width height)
    (sleep 5)
    ;; persistent Anonymous Thread
))


(tclip )

Expected: blue rectangle outline is not complete
Observed: despite clipping /without/ a rect all is drawn