make un/cache-image accept everything that image? accepts

I'm not sure if the precise details of how this function behaves
(specifically that it copies snips that aren't cacheable) is
important, but I suspect the implementation is more authoritative
that the docs in this case, so document what it really does.

closes PR 13956

original commit: fb91397d24551dbef0db7c40a9d12042e6c7326b

pkgs/gui-pkgs/gui-doc/mrlib/scribblings/image-core.scrbl

@@ -26,6 +26,11 @@ up an image.
 @defproc[(un/cache-image [image image?] [b any/c]) image?]{
   Returns an image that either caches its drawing in the 
   snip @method[snip% draw] method or doesn't, depending on @racket[b].
+  
+  Not all @racket[image?] values have special caching capabilities;
+  in those cases, this returns a copy of the value if it is a 
+  @racket[snip%]; otherwise it returns the value itself (if it 
+  isn't a @racket[snip%]).
 }
 
 @defproc[(compute-image-cache [image image?]) void?]{

pkgs/gui-pkgs/gui-lib/mrlib/image-core.rkt

@@ -84,10 +84,17 @@ has been moved out).
 
 (define (un/cache-image img bitmap-cache?)
   (unless (image? img)
-    (error 'un/cache-image "expected an image as the first argument, got ~e" img))
-  (define res (send img copy))
-  (send res set-use-bitmap-cache?! (and bitmap-cache? #t))
-  res)
+    (raise-argument-error 'un/cache-image
+                          "image?"
+                          0
+                          img bitmap-cache?))
+  (cond
+    [(is-a? img snip%)
+     (define res (send img copy))
+     (when (is-a? res image%)
+       (send res set-use-bitmap-cache?! (and bitmap-cache? #t)))
+     res]
+    [else img]))
 
 (define (compute-image-cache img)
   (unless (image? img)

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

@@ -0,0 +1,10 @@
+#lang racket/base
+
+(require rackunit 
+         mrlib/image-core
+         (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))