This fixes the following ASan detected leaks:

Direct leak of 88 byte(s) in 1 object(s) allocated from:
    #0 0x7f8c3455f330 in __interceptor_malloc (/lib/x86_64-linux-gnu/libasan.so.5+0xe9330)
    #1 0x7f8c33c60906 in wl_event_loop_add_timer ../../git/wayland/src/event-loop.c:571
    #2 0x7f8c2ff98f46 in shell_fade_init ../../git/weston/desktop-shell/shell.c:4211
    #3 0x7f8c2ff9e1da in wet_shell_init ../../git/weston/desktop-shell/shell.c:5266
    #4 0x7f8c3443ede5 in wet_load_shell ../../git/weston/compositor/main.c:956
    #5 0x7f8c3444fdb9 in wet_main ../../git/weston/compositor/main.c:3434
    #6 0x55878ad3bfc6 in execute_compositor ../../git/weston/tests/weston-test-fixture-compositor.c:432
    #7 0x55878ad3f9fa in weston_test_harness_execute_as_client ../../git/weston/tests/weston-test-runner.c:528
    #8 0x55878ad2fbd6 in fixture_setup ../../git/weston/tests/viewporter-test.c:46
    #9 0x55878ad2fc58 in fixture_setup_run_ ../../git/weston/tests/viewporter-test.c:48
    #10 0x55878ad3ffaf in main ../../git/weston/tests/weston-test-runner.c:661
    #11 0x7f8c340b409a in __libc_start_main ../csu/libc-start.c:308
    #12 0x55878ad2f769 in _start (/home/pq/build/weston-meson/tests/test-viewporter+0xc769)

Indirect leak of 856 byte(s) in 1 object(s) allocated from:
    #0 0x7f8c3455f518 in calloc (/lib/x86_64-linux-gnu/libasan.so.5+0xe9518)
    #1 0x7f8c33c99b73 in zalloc ../../git/weston/include/libweston/zalloc.h:38
    #2 0x7f8c33c9cfb1 in weston_surface_create ../../git/weston/libweston/compositor.c:574
    #3 0x7f8c2ff98230 in shell_fade_create_surface_for_output ../../git/weston/desktop-shell/shell.c:4059
    #4 0x7f8c2ff98df6 in shell_fade_init ../../git/weston/desktop-shell/shell.c:4202
    #5 0x7f8c2ff9e1da in wet_shell_init ../../git/weston/desktop-shell/shell.c:5266
    #6 0x7f8c3443ede5 in wet_load_shell ../../git/weston/compositor/main.c:956
    #7 0x7f8c3444fdb9 in wet_main ../../git/weston/compositor/main.c:3434
    #8 0x55878ad3bfc6 in execute_compositor ../../git/weston/tests/weston-test-fixture-compositor.c:432
    #9 0x55878ad3f9fa in weston_test_harness_execute_as_client ../../git/weston/tests/weston-test-runner.c:528
    #10 0x55878ad2fbd6 in fixture_setup ../../git/weston/tests/viewporter-test.c:46
    #11 0x55878ad2fc58 in fixture_setup_run_ ../../git/weston/tests/viewporter-test.c:48
    #12 0x55878ad3ffaf in main ../../git/weston/tests/weston-test-runner.c:661
    #13 0x7f8c340b409a in __libc_start_main ../csu/libc-start.c:308
    #14 0x55878ad2f769 in _start (/home/pq/build/weston-meson/tests/test-viewporter+0xc769)

Indirect leak of 608 byte(s) in 1 object(s) allocated from:
    #0 0x7f8c3455f518 in calloc (/lib/x86_64-linux-gnu/libasan.so.5+0xe9518)
    #1 0x7f8c33c99b73 in zalloc ../../git/weston/include/libweston/zalloc.h:38
    #2 0x7f8c33c9bed5 in weston_view_create ../../git/weston/libweston/compositor.c:365
    #3 0x7f8c2ff98251 in shell_fade_create_surface_for_output ../../git/weston/desktop-shell/shell.c:4063
    #4 0x7f8c2ff98df6 in shell_fade_init ../../git/weston/desktop-shell/shell.c:4202
    #5 0x7f8c2ff9e1da in wet_shell_init ../../git/weston/desktop-shell/shell.c:5266
    #6 0x7f8c3443ede5 in wet_load_shell ../../git/weston/compositor/main.c:956
    #7 0x7f8c3444fdb9 in wet_main ../../git/weston/compositor/main.c:3434
    #8 0x55878ad3bfc6 in execute_compositor ../../git/weston/tests/weston-test-fixture-compositor.c:432
    #9 0x55878ad3f9fa in weston_test_harness_execute_as_client ../../git/weston/tests/weston-test-runner.c:528
    #10 0x55878ad2fbd6 in fixture_setup ../../git/weston/tests/viewporter-test.c:46
    #11 0x55878ad2fc58 in fixture_setup_run_ ../../git/weston/tests/viewporter-test.c:48
    #12 0x55878ad3ffaf in main ../../git/weston/tests/weston-test-runner.c:661
    #13 0x7f8c340b409a in __libc_start_main ../csu/libc-start.c:308
    #14 0x55878ad2f769 in _start (/home/pq/build/weston-meson/tests/test-viewporter+0xc769)

They were found with:

	ASAN_OPTIONS=fast_unwind_on_malloc=0,malloc_context_size=50 \
	LSAN_OPTIONS=suppressions=/home/pq/git/weston/.gitlab-ci/leak-sanitizer.supp \
	./tests/test-viewporter test_viewporter_double_create

Turns out shell_destroy() had an open-coded and outdated copy of the
tear-down sequence, so fixing the leaks in only handle_output_destroy()
was not enough.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>