[racket] rotate an image using 2htdp/image

@Robby Findler : I see what you mean. But in this example given by Paul
Ojanen
 (require 2htdp/image)
(require 2htdp/universe)

(define BACKGROUND (rectangle 100 100 "outline" "white"))
(define TRI (triangle 35 "solid" "blue"))

(define (render ang)
 (place-image (rotate ang TRI) 20 20 BACKGROUND))

(define (tock ang)
 (modulo (+ ang 10) 360))

(big-bang 0 (on-draw render) (on-tick tock))

The triangle is not being consistent by keeping its center point on the
center of BACKGROUND.

Hopefully, this makes sense.
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