Don’t draw diagonal lines instead of points at high scaling factors

draw-point (and draw-line with a line length of zero) actually draws a
very small line at a 45° angle, which forces Cairo to draw the line cap.
But at large scaling factors, this tiny line stops looking like a point
and starts looking like an actual line, which isn’t good.

This commit adjusts the length of the tiny line based on the current
scaling factor, so it will still look like a point even when scaled up.

draw-lib/racket/draw/private/dc.rkt

@@ -1158,28 +1158,30 @@
     (def/public (draw-ellipse [real? x] [real? y] [nonnegative-real? width] [nonnegative-real? height])
       (do-draw-arc 'draw-ellipse x y width height 0 2pi))
 
+    (define/private (do-draw-point who x y)
+      (with-cr
+        (check-ok who)
+        cr
+        (cairo_new_path cr)
+        (let ([x (align-x x)]
+              [y (align-y y)])
+          (cairo_move_to cr x y)
+          (cairo_line_to cr (+ x (/ 0.1 effective-scale-x)) (+ y (/ 0.1 effective-scale-y)))
+          (draw cr #f #t))))
+
     (def/public (draw-line [real? x1] [real? y1] [real? x2] [real? y2])
-      (let ([dot (if (and (= x1 x2) (= y1 y2))
-                     0.1
-                     0)])
-        (with-cr
-         (check-ok 'draw-line)
-         cr
-         (cairo_new_path cr)
-         (cairo_move_to cr (align-x x1) (align-y y1))
-         (cairo_line_to cr (+ (align-x x2) dot) (+ (align-y y2) dot))
-         (draw cr #f #t))))
+      (if (and (= x1 x2) (= y1 y2))
+          (do-draw-point 'draw-line x1 y1)
+          (with-cr
+            (check-ok 'draw-line)
+            cr
+            (cairo_new_path cr)
+            (cairo_move_to cr (align-x x1) (align-y y1))
+            (cairo_line_to cr (align-x x2) (align-y y2))
+            (draw cr #f #t))))
 
     (def/public (draw-point [real? x] [real? y])
-      (with-cr
-       (check-ok 'draw-point)
-       cr
-       (cairo_new_path cr)
-       (let ([x (align-x x)]
-             [y (align-y y)])
-         (cairo_move_to cr x y)
-         (cairo_line_to cr (+ 0.1 x) (+ 0.1 y))
-       (draw cr #f #t))))
+      (do-draw-point 'draw-point x y))
 
     (def/public (draw-lines [(make-alts (make-list point%) list-of-pair-of-real?) pts]
                             [real? [x 0.0]] [real? [y 0.0]])