csd-background: Remove decoration from desktop window (linuxmint#434)

Without this, window decoration is drawn and the background is shifted
in XWayland if `GTK_CSD=1` environment variable is set.

Author: City-busz

plugins/background/monitor-background.c

@@ -63,6 +63,7 @@ build_monitor_background (MonitorBackground *mb)
 
     mb->window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
     gtk_window_set_type_hint (GTK_WINDOW (mb->window), GDK_WINDOW_TYPE_HINT_DESKTOP);
+    gtk_window_set_decorated (GTK_WINDOW (mb->window), FALSE);
 
     // Set keep below so muffin recognizes the backgrounds and keeps them
     // at the bottom of the bottom window layer (under file managers, etc..)