libweston: move weston_compositor_shutdown call out of backends

If we want to support multiple backends, the compositor must take care
to call this once, at the appropriate moment, so stop letting the
backends handle compositor shutdown themselves.

Move the weston_compositor_shutdown() calls from the backend::destroy
callbacks into weston_compositor_destroy() and the calls in the backend
creation error paths into weston_compositor_load_backend().

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>