canvas% on-scroll: call in non-atomic mode

Add a missing exit from atomic mode on the way to an overidden
`on-scroll` method.

Closes PR 15068

gui-lib/mred/private/wxcanvas.rkt

@@ -62,7 +62,7 @@
                    (lambda (e)
                      (let ([mred (get-mred)])
                        (if mred
-                           (send mred on-scroll e)
+                           (as-exit (lambda () (send mred on-scroll e)))
                            (as-exit (lambda () (super on-scroll e)))))))]
        [on-paint (entry-point
                   (lambda ()