x11-display: Set the correct default Xcursor theme size

Under Xorg the cursor size preference was pre-scaled originating from
gtk, while with Wayland it came directly from GSettings remaining
unscaled. Under Xwayland this caused the X11 display code to set the
wrong size with HiDPI configurations, which was often later overridden
by the equivalent code in gtk, but not always.

Fix this by always having the cursor size preference unscaled, scaling
the size correctly where it's used, depending on how it's used.

https://bugzilla.gnome.org/show_bug.cgi?id=759538

src/core/display.c

@@ -676,6 +676,13 @@ on_startup_notification_changed (MetaStartupNotification *sn,
   g_signal_emit_by_name (display, "startup-sequence-changed", sequence);
 }
 
+static void
+on_ui_scaling_factor_changed (MetaSettings *settings,
+                              MetaDisplay  *display)
+{
+  meta_display_reload_cursor (display);
+}
+
 /**
  * meta_display_open:
  *
@@ -697,6 +704,7 @@ meta_display_open (void)
   Window old_active_xwindow = None;
   MetaBackend *backend = meta_get_backend ();
   MetaMonitorManager *monitor_manager;
+  MetaSettings *settings;
 
   g_assert (the_display == NULL);
   display = the_display = g_object_new (META_TYPE_DISPLAY, NULL);
@@ -755,6 +763,10 @@ meta_display_open (void)
   g_signal_connect (monitor_manager, "monitors-changed-internal",
                     G_CALLBACK (on_monitors_changed_internal), display);
 
+  settings = meta_backend_get_settings (backend);
+  g_signal_connect (settings, "ui-scaling-factor-changed",
+                    G_CALLBACK (on_ui_scaling_factor_changed), display);
+
   meta_display_set_cursor (display, META_CURSOR_DEFAULT);
 
   display->stack = meta_stack_new (display);