mrlib/gif: don't depend on racket/gui/base

The only identifier it had been importing from racket/gui/base was bitmap%, which can be imported from racket/draw instead.

Author: AlexKnauth

gui-lib/mrlib/gif.rkt

@@ -2,7 +2,7 @@
 
 ;; A library for creating gifs out of bitmap%s
 
-(require racket/gui/base
+(require racket/draw
          racket/class
          racket/list
          net/gifwrite