compositor: Don't ignore --use-pixman for Wayland backend

We loaded the use-pixman configuration value from both the command line
and the configuration file, but completely ignored the former. Make sure
we actually use both.

Tested with all permutations of config/command line.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
(cherry picked from commit 8a4585c2)