Add support for direct file reading and writing in wayland-scanner.

Add support for direct file reading and writing in wayland-scanner.

Signed-off-by: Jussi Pakkanen <jpakkane@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Acked-by: David Fort <rdp.effort@gmail.com>