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

Otherwise 'log_extensions()' will not know how to 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 31 seconds (queued for 14 seconds)
Status Job ID Name Coverage
  Container Prep
passed #477021
container_prep

00:00:29

 
  Build
passed #477022
build-native-meson

00:01:02