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>
2 jobs for wayland-backend-fix-use-pixman in 2 minutes and 5 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #200205
build-native-autotools

00:02:05

passed #200206
build-native-meson

00:01:36