backend: Get the UI scaling factor from the logical monitors

Instead of looking at the GTK+ settings, check the logical monitor
state and determine the UI scaling factor given the maximum logical
monitor scale. This is only enabled when the monitor config manager
feature is enabled, as only then can a scale be explicitly configured.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/backends/meta-backend.c

@@ -97,6 +97,8 @@ struct _MetaBackendPrivate
 
   MetaPointerConstraint *client_pointer_constraint;
   MetaDnd *dnd;
+
+  int ui_scaling_factor;
 };
 typedef struct _MetaBackendPrivate MetaBackendPrivate;
 
@@ -151,6 +153,25 @@ center_pointer (MetaBackend *backend)
                              primary->rect.y + primary->rect.height / 2);
 }
 
+static gboolean
+meta_backend_update_ui_scaling_factor (MetaBackend *backend)
+{
+  MetaBackendPrivate *priv = meta_backend_get_instance_private (backend);
+  int ui_scaling_factor;
+
+  ui_scaling_factor = meta_backend_get_ui_scaling_factor (backend);
+
+  if (ui_scaling_factor != priv->ui_scaling_factor)
+    {
+      priv->ui_scaling_factor = ui_scaling_factor;
+      return TRUE;
+    }
+  else
+    {
+      return FALSE;
+    }
+}
+
 void
 meta_backend_monitors_changed (MetaBackend *backend)
 {
@@ -170,6 +191,9 @@ meta_backend_monitors_changed (MetaBackend *backend)
           !meta_monitor_manager_is_headless (monitor_manager))
         center_pointer (backend);
     }
+
+  if (meta_backend_update_ui_scaling_factor (backend))
+    meta_backend_notify_ui_scaling_factor_changed (backend);
 }
 
 void
@@ -339,6 +363,8 @@ meta_backend_real_post_init (MetaBackend *backend)
 
   meta_backend_sync_screen_size (backend);
 
+  meta_backend_update_ui_scaling_factor (backend);
+
   priv->cursor_renderer = META_BACKEND_GET_CLASS (backend)->create_cursor_renderer (backend);
 
   priv->device_monitors =
@@ -983,6 +1009,7 @@ xft_dpi_changed (GtkSettings *settings,
                  GParamSpec  *pspec,
                  MetaBackend *backend)
 {
+  meta_backend_update_ui_scaling_factor (backend);
   meta_backend_notify_ui_scaling_factor_changed (backend);
 }
 
@@ -1070,13 +1097,41 @@ meta_backend_notify_keymap_layout_group_changed (MetaBackend *backend,
                  locked_group);
 }
 
+static int
+calculate_ui_scaling_factor (MetaBackend *backend)
+{
+  MetaMonitorManager *monitor_manager =
+    meta_backend_get_monitor_manager (backend);
+  GList *logical_monitors;
+  GList *l;
+  int max_scale = 1;
+
+  logical_monitors =
+    meta_monitor_manager_get_logical_monitors (monitor_manager);
+  for (l = logical_monitors; l; l = l->next)
+    {
+      MetaLogicalMonitor *logical_monitor = l->data;
+
+      max_scale = MAX (logical_monitor->scale, max_scale);
+    }
+
+  return max_scale;
+}
+
 int
 meta_backend_get_ui_scaling_factor (MetaBackend *backend)
 {
   if (meta_is_stage_views_scaled ())
-    return 1;
+    {
+      return 1;
+    }
   else
-    return meta_theme_get_window_scaling_factor ();
+    {
+      if (meta_is_monitor_config_manager_enabled ())
+        return calculate_ui_scaling_factor (backend);
+      else
+        return meta_theme_get_window_scaling_factor ();
+    }
 }
 
 void