GtkWindow: Don't needlessly resize popovers

GtkWindow: Don't needlessly resize popovers

Check whether the given popover even changed size in
_gtk_window_set_popover_position. If not, just move its GdkWindow
without calling gtk_widget_queue_resize. Using popover_get_rect here is
still relatively costly, but popover_size_allocate would be doing that
anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=755435