ui: Fix default window_id value

ui: Fix default window_id value

./chardev/baum.c expects the default window_id value to be -1, and not 0
which could be confused with a proper window id (when numbered from 0 by
the ui backend).

This fixes getting Braille output with the curses and gtk frontends.

Fixes: f29b3431f62 ("console: move window ID code from baum to sdl")
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200914100637.eeommoflirxrgaeh@function>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>