compositor: Return the number of bytes written as to format properly

Otherwise 'log_extensions()' will not know how to properly format the
data.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Reported-by: Pekka Paalanen <pekka.paalanen@collabora.com>
2 jobs for fix-log-extensions in 1 minute and 24 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Container Prep
passed #513821
container_prep

00:00:27

 
  Build
passed #513822
build-native-meson

00:00:56