avoid spurious screen refresh on text-measure operations

The drawing layer now indicates whether drawing content was
changed as it releases a drawing handle.

gui-lib/info.rkt

@@ -6,7 +6,7 @@
                "data-lib"
                ["base" #:version "6.4"]
                "syntax-color-lib"
-               ["draw-lib" #:version "1.11"]
+               ["draw-lib" #:version "1.13"]
                ["snip-lib" #:version "1.2"]
                "wxme-lib"
                "pict-lib"

gui-lib/mred/private/wx/common/backing-dc.rkt

@@ -134,6 +134,9 @@
       (when (zero? flush-suspends)
         (queue-backing-flush)))
 
+    (define/override (release-unchanged-cr cr)
+      (void))
+
     (define/override (erase)
       (super erase)
       (when (= (get-clear-operator)