compositor: Support xdg_output_unstable_v1

The xdg-output resources are listed in each head struct. They become idle when
the respective weston_output has been removed again. The client is supposed to
destroy them explicitly afterwards.

After starting an XWayland client xrandr displays the logical size as expected.

Signed-off-by: Roman Gilg <subdiff@gmail.com>
2 jobs for xdgOutput in 2 minutes (queued for 1 second)
Status Job ID Name Coverage
  Container Prep
passed #204154
container_prep

00:01:24

 
  Build
passed #204155
build-native-meson

00:00:35