wlt: toolkit: flush display after wl_display_dispatch_pending()

wlterm doesn't work with current Wayland/Weston; the events are never
sent and the window doesn't come up. We need to flush the display, as
per wl_display_dispatch_pending(3):

    To proper integrate the wayland display fd into a main loop, the
    client should always call wl_display_dispatch_pending() and then
    wl_display_flush() prior to going back to sleep.

Signed-off-by: Ran Benita <ran234@gmail.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>