fix definitely-same-image? for the image-snip case

closes PR 14951

gui-lib/mrlib/image-core.rkt

@@ -471,10 +471,10 @@ has been moved out).
      #f]
     [else
      (define bm1 (if (is-a? i1 image-snip%)
-                     (send i1 get-image)
+                     (send i1 get-bitmap)
                      i2))
      (define bm2 (if (is-a? i2 image-snip%)
-                     (send i2 get-image)
+                     (send i2 get-bitmap)
                      i2))
      (eq? bm1 bm2)]))
 

gui-test/mrlib/tests/image-core.rkt

@@ -2,9 +2,22 @@
 
 (require rackunit 
          mrlib/image-core
+         racket/class
          (only-in racket/gui/base make-bitmap))
 
 ;; just check there is no error
 (check-equal? (begin (un/cache-image (make-bitmap 1 1) #t)
                      (void))
-              (void))
+              (void))
+
+(check-equal?
+ (let ([b (make-bitmap 1 1)])
+   (definitely-same-image? b b))
+ #t)
+
+(check-equal?
+ (let* ([b (make-bitmap 1 1)])
+   (define s1 (make-object image-snip% b))
+   (define s2 (make-object image-snip% b))
+   (definitely-same-image? s1 s2))
+ #t)