`draw-bitmap` ignores smoothing when the bitmap has a backing scale greater than 1.0 (at least on macOS)

On my machine, macOS 10.13.6, the draw-bitmap method of bitmap-dc% appears to ignore the current smoothing setting if the bitmap to be drawn has a backing scale greater than 1.0. This program reproduces the problem:

#lang racket

(require racket/draw)

(define (make-point-bitmap backing-scale)
  (define bmp (make-bitmap 3 3 #:backing-scale backing-scale))
  (define dc (new bitmap-dc% [bitmap bmp]))
  (send dc set-smoothing 'unsmoothed)
  (send dc draw-point 1.5 1.5)
  bmp)

(define (blow-up bmp scale)
  (define bmp* (make-bitmap (* (send bmp get-width) scale)
                            (* (send bmp get-height) scale)))
  (define dc (new bitmap-dc% [bitmap bmp*]))
  (send dc set-smoothing 'unsmoothed)
  (send dc set-scale scale scale)
  (send dc draw-bitmap bmp 0 0)
  bmp*)

(send (blow-up (make-point-bitmap 1.0) 100) save-file "blown-up-1.png" 'png)
(send (blow-up (make-point-bitmap 2.0) 100) save-file "blown-up-2.png" 'png)

The program produces the following images as output:

I would expect them to be the same, but the second is antialiased while the first one isn’t.

Start by running the provided Racket reproduction on macOS and comparing the two generated PNGs. Trace the draw-bitmap behavior for bitmap-dc%, set-smoothing, and backing-scale; done means the two outputs respect the same unsmoothed setting when their backing scales differ.