Destroying the compositor after destroying the log scope will not print
out the messages in the tear down/clean-up phase of the compositor, so
add a new tear_down function which allows keeping a valid reference to
the compositor. This way we can destroy the compositor before destroying
the scope and keep the debug messages.

While at it remove the log context destroy part from the clean-up
of the compositor and make it stand on its own.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>