[gimp] app: check if display is X11 before gdk_x11_display_get_user_time().

[Jehan <jehanp src gnome org>]

commit 66a952df9fb18a071661a8a6b98be49bdc98b9c1
Author: Jehan <jehan girinstud io>
Date:   Mon Jun 15 21:34:54 2020 +0200

    app: check if display is X11 before gdk_x11_display_get_user_time().
    
    Fixes:
    > GLib-GObject-WARNING **: 20:51:24.156: invalid cast from 'GdkWaylandDisplay' to 'GdkX11Display'

 app/gui/gui-vtable.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/app/gui/gui-vtable.c b/app/gui/gui-vtable.c
index 4545621593..b585a19c38 100644
--- a/app/gui/gui-vtable.c
+++ b/app/gui/gui-vtable.c
@@ -314,9 +314,11 @@ static guint32
 gui_get_user_time (Gimp *gimp)
 {
 #ifdef GDK_WINDOWING_X11
-  return gdk_x11_display_get_user_time (gdk_display_get_default ());
+  if (GDK_IS_X11_DISPLAY (gdk_display_get_default ()))
+    return gdk_x11_display_get_user_time (gdk_display_get_default ());
 #endif
-  return 0;
+
+  return gtk_get_current_event_time ();
 }
 
 static GFile *