tiling: keep track of the monitor where a window was tiled

meta_window_get_current_tile_area() computes the area where the tiled window
should be based on the current pointer position but that's only meaningful
when the user is actually dragging the window.

When running the tiling constrain the pointer might be on other monitor and at
that point the window jumps to this other monitor.

https://bugzilla.gnome.org/show_bug.cgi?id=642580

src/core/display-private.h

@@ -189,6 +189,7 @@ struct _MetaDisplay
   int         grab_anchor_root_y;
   MetaRectangle grab_anchor_window_pos;
   MetaTileMode  grab_tile_mode;
+  int           grab_tile_monitor_number;
   int         grab_latest_motion_x;
   int         grab_latest_motion_y;
   gulong      grab_mask;