cache bitmap and dc for snip sizing

svn: r4708

collects/lang/private/imageeq.ss

@@ -11,15 +11,17 @@
   (define (image? a)
     (or (is-a? a image-snip%)
         (is-a? a cache-image-snip%)))
+
+  (define size-dc (delay (make-object bitmap-dc% (make-object bitmap% 1 1))))
   
   (define (snip-size a)
     (cond
       [(is-a? a cache-image-snip%)
        (send a get-size)]
       [else
-       (let ([dc (make-object bitmap-dc% (make-object bitmap% 1 1))]
-             [wb (box 0)]
-             [hb (box 0)])
+       (let* ([dc (force size-dc)]
+              [wb (box 0)]
+              [hb (box 0)])
          (send a get-extent dc 0 0 wb hb #f #f #f #f)
          (values (unbox wb) 
                  (unbox hb)))]))

collects/lang/private/teachhelp.ss

@@ -28,16 +28,16 @@
 		   (syntax args))
 	     stx)]
 	   [id
-	    (syntax-property
-	     (datum->syntax-object
-	      check-proc
-	      (list check-proc 
-		    (list 'quote (syntax id))
-		    tmp-id)
-	      stx)
+            (syntax-property
+             (datum->syntax-object
+              check-proc
+              (list check-proc 
+                    (list 'quote (syntax id))
+                    tmp-id)
+              stx)
              'stepper-skipto
              '(syntax-e cdr syntax-e cdr cdr car))]))))) ; this may make other stepper-skipto annotations obsolete.
-
+    
   (define (appropriate-use what)
     (case what
      [(constructor)