fix continuation-mark splicing across composable combinations

Although splicing was set up for applying a composable
comtinuation to most kinds of continuations, it was not
set up right for applying a composable continaution in tail
position for a just-applied composable continuation.

Thanks to Spencer Florence for the report and example.

pkgs/racket-test-core/tests/racket/prompt-tests.rktl

@@ -2395,3 +2395,32 @@
          (λ () #f))
      (λ (x) x)))))
 
+;; ----------------------------------------
+
+(test
+ '(2)
+ 'dont-double-continuation-mark
+ (let ()
+   (define tag (make-continuation-prompt-tag))
+
+   (define k
+     (call-with-continuation-prompt
+      (lambda ()
+        (with-continuation-mark 'test 2
+                                (#%app (call-with-composable-continuation (lambda (k) (abort-current-continuation tag k)) tag))))
+      tag
+      values))
+
+   (define (compose-continuations k1 k2)
+     (call-with-continuation-prompt
+      (lambda ()
+        (k1 (lambda ()
+              (k2 (lambda () (#%app (call-with-composable-continuation (lambda (k) (abort-current-continuation tag k)) tag)))))))
+      tag
+      values))
+
+   (continuation-mark-set->list
+    (continuation-marks (let ([c1 (compose-continuations k k)])
+                          (compose-continuations c1 c1))
+                        tag)
+    'test)))

racket/src/racket/src/fun.c

@@ -5468,6 +5468,7 @@ static void restore_continuation(Scheme_Cont *cont, Scheme_Thread *p, int for_pr
         /* Prune resume_mc continuation marks that have replacements
            in the deepest frame of cont, and add extra_marks */
         prune_cont_marks(resume_mc, cont, extra_marks);
+        p->cont_mark_pos_bottom = cont->cont_mark_pos_bottom;
       }
       
       mc = clone_meta_cont(cont->meta_continuation, NULL, -1, NULL, NULL, resume_mc, 0);
@@ -5624,6 +5625,8 @@ static void restore_continuation(Scheme_Cont *cont, Scheme_Thread *p, int for_pr
       }
     }
 
+    meta_prompt->boundary_mark_pos = cont->cont_mark_pos_bottom; /* for mark splicing */
+
     p->meta_prompt = meta_prompt;
   }