`blur` does not accommodate the current backing scale

[lexi-lambda]

Since blur draws into a bitmap, it should be backing scale-aware. This program demonstrates the problem:

#lang racket

(require racket/gui/base
         pict
         pict/shadow)

(define bmp (make-bitmap 10 10 #:backing-scale 50))
(define dc (new bitmap-dc% [bitmap bmp]))
(draw-pict (blur (circle 8) 1) dc 1 1)

(define bmp-bytes (make-bytes (* 500 500 4)))
(send bmp get-argb-pixels 0 0 500 500 bmp-bytes #:unscaled? #t)
(define bmp* (make-bitmap 500 500))
(send bmp* set-argb-pixels 0 0 500 500 bmp-bytes)
(show-pict (bitmap bmp*))

It produces the following image as output:

…while a blur implementation that accommodates the current backing scale should produce this image, instead:

This example is obviously intentionally exaggerated, but this issue does mean that when rendering to HiDPI displays, blur will produce bitmaps at less than screen quality.