compositor: force fade-in on start

compositor: force fade-in on start

Weston initialises to faded-out state, which means all outputs are just
black, even if they render fine.

Previously the fade-in was triggered probably by some random input
event, since Weston was not guaranteed to wake up. Especially with the
drm backend, and the usual problem of not having permissions to input
devices, Weston would not fade in at all and appear broken.

Force Weston to fade in right after initialisation. This show that at
least rendering works, if it does not respond to any input.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>