wayland: Fix a race condition with xdg_popup resize

wayland: Fix a race condition with xdg_popup resize

When resizing an xdg_popup immediately after the initial mapping, there
is a race condition between the client and the compositor which is
processing the initial size given by the xdg_positioner, leading to the
xdg_popup to be eventually of the wrong size.

Only way to make sure the size is correct in that case is to hide and
show the window again. Considering this occurs before the initial
configure is processed, it should not be noticeable.

https://bugzilla.gnome.org/show_bug.cgi?id=772505