racket/draw: fix font metrics versus transformations

For a font with 'aligned hinting, the font map caches metric
information that depends on the destination transformation,
at least on Windows. Make the font-map cache sensitive to the
destination's current transformation.

This bug was exposed by support for DPI-aware GUIs on Windows,
but the problem was more general.

original commit: 9ee2bd9b6086c696c2be8f01a07db754e4406312

pkgs/gui-pkgs/gui-lib/mred/private/wx/common/backing-dc.rkt

@@ -183,8 +183,13 @@
     
     (define/override (init-cr-matrix cr)
       (unless (= backing-scale 1.0)
-	      (cairo_scale cr backing-scale backing-scale))
+        (cairo_scale cr backing-scale backing-scale))
       (super init-cr-matrix cr))
+
+    (define/override (init-effective-matrix mx)
+      (unless (= backing-scale 1.0)
+        (cairo_matrix_scale mx backing-scale backing-scale))
+      (super init-effective-matrix mx))
     
     (define/override (reset-config s)
       (set! backing-scale s)

pkgs/gui-pkgs/gui-test/tests/gracket/font-maps.rkt

@@ -0,0 +1,37 @@
+#lang racket
+(require racket/draw)
+
+;; Check for pollution of font metrics from differently
+;; scaled contexts.
+
+(define font (make-font #:face "Times"))
+
+;; Running `go` might affect the result of `go2`
+(define (go)
+  (define bm (make-bitmap 1 1))
+  (send (send bm make-dc) get-text-extent
+	"Extra regexp"
+	font
+	#t))
+
+;; `go2` is like `go`, but for a different scale
+(define (go2)
+  (define bm2 (make-platform-bitmap 1 1))
+  (define dc (send bm2 make-dc))
+  (send dc scale 1.25 1.25)
+  (send dc get-text-extent
+	"Extra regexp"
+	font
+	#t))
+
+;; Running `go2` again in a separate place might produce
+;; results unaffected by `go`:
+(define (go2/p)
+  (place pch (place-channel-put pch (call-with-values go2 list))))
+
+(module+ test
+  (call-with-values go void)
+  (define l1 (call-with-values go2 list))
+  (define l2 (sync (go2/p)))
+  (unless (equal? l1 l2)
+    (error 'different "~s ~s" l1 l2)))