main: Use the output handler to destroy to black view

Use the specific output destroyer handler, instead of destroying it in
the handler itself. This way we can still clean-up and avoid any leaks,
and still not trip ourselves causing a use-after-free, when the output
is destroyed by libweston.

Bug-AGL: SPEC-3630, SPEC-3672

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Reported-by: Walter Lozano <walter.lozano@collabora.com>
Change-Id: I11916245b85d2541286ee438284601b466dc20c2