fix priority of refresh for SGL example

While it looks this example was always broken in a sense, the
`racket/gui' make the problem more prominent.

Closes PR 12662

collects/sgl/examples/gl-frame.rkt

@@ -70,7 +70,7 @@
              (set! init? #t))
            (gl-draw)
            (swap-gl-buffers)))
-        (refresh))
+        (queue-callback (lambda () (refresh)) #f))
       
       (define/override (on-size w h)
         (with-gl-context