[gnome-flashback] root-background: get display size from monitors

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 1915a794ae255dd04344081abc024b53ea046640
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Mon Nov 25 01:15:17 2019 +0200

    root-background: get display size from monitors

 .../libroot-background/gf-root-background.c        | 32 ++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)
---
diff --git a/gnome-flashback/libroot-background/gf-root-background.c 
b/gnome-flashback/libroot-background/gf-root-background.c
index 5856fc8..a0f9d14 100644
--- a/gnome-flashback/libroot-background/gf-root-background.c
+++ b/gnome-flashback/libroot-background/gf-root-background.c
@@ -39,6 +39,34 @@ struct _GfRootBackground
 
 G_DEFINE_TYPE (GfRootBackground, gf_root_background, G_TYPE_OBJECT)
 
+static void
+get_display_size (GdkDisplay *display,
+                  int        *width,
+                  int        *height)
+{
+  GdkRectangle rect;
+  int n_monitors;
+  int i;
+
+  rect = (GdkRectangle) {};
+  display = gdk_display_get_default ();
+  n_monitors = gdk_display_get_n_monitors (display);
+
+  for (i = 0; i < n_monitors; i++)
+    {
+      GdkMonitor *monitor;
+      GdkRectangle geometry;
+
+      monitor = gdk_display_get_monitor (display, i);
+
+      gdk_monitor_get_geometry (monitor, &geometry);
+      gdk_rectangle_union (&rect, &geometry, &rect);
+    }
+
+  *width = rect.width;
+  *height = rect.height;
+}
+
 static void
 set_background (GfRootBackground *self)
 {
@@ -52,8 +80,8 @@ set_background (GfRootBackground *self)
   display = gdk_display_get_default ();
   screen = gdk_display_get_default_screen (display);
   root = gdk_screen_get_root_window (screen);
-  width = gdk_window_get_width (root);
-  height = gdk_window_get_height (root);
+
+  get_display_size (display, &width, &height);
 
   surface = gf_background_surface_create (display,
                                           self->bg,