[gtk+] Implement gdk_screen_get_monitor_scale_factor generically

[Matthias Clasen <matthiasc src gnome org>]

commit 21bdf617cea13d0625e88d9cb0d03bd0df169e2d
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Sep 13 10:45:06 2016 -0400

    Implement gdk_screen_get_monitor_scale_factor generically
    
    This was forgotten when the other screen monitor apis were
    ported to GdkMonitor.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=771349

 gdk/gdkscreen.c |    9 ++++-----
 1 files changed, 4 insertions(+), 5 deletions(-)
---
diff --git a/gdk/gdkscreen.c b/gdk/gdkscreen.c
index 85cb93f..2249e7e 100644
--- a/gdk/gdkscreen.c
+++ b/gdk/gdkscreen.c
@@ -1095,16 +1095,15 @@ gint
 gdk_screen_get_monitor_scale_factor (GdkScreen *screen,
                                      gint       monitor_num)
 {
-  GdkScreenClass *screen_class;
+  GdkMonitor *monitor;
 
   g_return_val_if_fail (GDK_IS_SCREEN (screen), 1);
   g_return_val_if_fail (monitor_num >= 0, 1);
   g_return_val_if_fail (monitor_num < gdk_screen_get_n_monitors (screen), 1);
 
-  screen_class = GDK_SCREEN_GET_CLASS (screen);
+  monitor = get_monitor (screen, monitor_num);
 
-  if (screen_class->get_monitor_scale_factor)
-    return screen_class->get_monitor_scale_factor (screen, monitor_num);
+  g_return_val_if_fail (monitor != NULL, 1);
 
-  return 1;
+  return gdk_monitor_get_scale_factor (monitor);
 }