backend-wayland: fix --fullscreen

Don't forget to transfer the fullscreen setting from the config to the backend.
Without this, weston tries to resize the window with the windowed output API but
that is not registered with new_config.fullscreen == true.

This code was accidentally lost in 0a5bb7ac
("backend-wayland: Use renderer enum type for config selection"). So just
restore it.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>