[gtk/wip/muktupavels/root-surface-scale] gdkscreen-x11: update device scale also for root window

[Alberts Muktupāvels <muktupavels src gnome org>]

commit d739367a7332a4eaf0af63a174d313857b24475d
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Mon Nov 25 15:28:20 2019 +0200

    gdkscreen-x11: update device scale also for root window

 gdk/x11/gdkscreen-x11.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/gdk/x11/gdkscreen-x11.c b/gdk/x11/gdkscreen-x11.c
index 8939ae9fac..a4b2ebc18a 100644
--- a/gdk/x11/gdkscreen-x11.c
+++ b/gdk/x11/gdkscreen-x11.c
@@ -1007,6 +1007,10 @@ _gdk_x11_screen_set_window_scale (GdkX11Screen *x11_screen,
   root = x11_screen->root_window;
   GDK_WINDOW_IMPL_X11 (root->impl)->window_scale = scale;
 
+  if (GDK_WINDOW_IMPL_X11 (root->impl)->cairo_surface)
+    cairo_surface_set_device_scale (GDK_WINDOW_IMPL_X11 (root->impl)->cairo_surface,
+                                    scale, scale);
+
   toplevels = gdk_screen_get_toplevel_windows (GDK_SCREEN (x11_screen));
 
   for (l = toplevels; l != NULL; l = l->next)