window: Set fall-back tile monitor if not set

When tiling, we want to set the tile monitor. To not have to do this
from the call site, make meta_window_tile() fall back to the current
monitor if nothing set it prior to the call.

This will make it more convenient for test cases to test tiling
behavior.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1171

src/core/window.c

@@ -3156,6 +3156,10 @@ meta_window_tile (MetaWindow   *window,
       window->tile_monitor_number = -1;
       return;
     }
+  else
+    {
+      window->tile_monitor_number = window->monitor->number;
+    }
 
   if (window->tile_mode == META_TILE_MAXIMIZED)
     directions = META_MAXIMIZE_BOTH;