compositor: Properly dispose of fullscreen views at shutdown

compositor: Properly dispose of fullscreen views at shutdown

While we can have multiple outputs, each with its own fullscreen view,
we have a layer-per-output hence we can't really call layer_fini()
in the output destruction as that what would result in an invalid
link access.

Handle those views destruction and with it, the fullscreen layer fini,
in the finalize bit where all others are handled as well.

This fixes a crash at shutdown, which could be seen when using multiple
outputs, and probably quite problematic when handling a restart.

Fixes: 43bdf9a42bac33df174d6ac7ff1ab2e15441c7b5

Bug-AGL: SPEC-4420
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I773890a24ad8ca2556c1d4ddda7f37ebae02c9de