trigger Slideshow printing via `executable-yield-handler'

Closes PR 11595

collects/slideshow/start.rkt

@@ -65,8 +65,7 @@
 			    (lambda (who where-name where-port-num mode)
 			      (error 'slideshow
 				     "slide program attempted to make a network connection")))))
-    (dynamic-require (path->complete-path content) #f)
-    (done-making-slides))
+    (dynamic-require (path->complete-path content) #f))
 
   (when (file-to-load)
     (load-content (string->path (file-to-load))))

collects/slideshow/viewer.rkt

@@ -1223,6 +1223,14 @@
 	(when config:printing?
 	  (do-print)))
 
+      (when config:printing?
+        ;; Just before exiting normally, print the slides:
+        (let ([h (executable-yield-handler)])
+          (executable-yield-handler
+           (lambda (v)
+             (viewer:done-making-slides)
+             (h v)))))
+
       (let ([eh (uncaught-exception-handler)])
 	(uncaught-exception-handler
 	 (lambda (exn)