Giulio Camuffo authored Apr 29, 2016 and Pekka Paalanen committed May 10, 2016

Preparing for libweston and for the separation of the code base into
libweston vs. weston the compositor, we must remove all uses
weston_config structures from the backends. We have decided that all
option and config input happens in the compositor (main.c), and
configuration is passed in for the backends as structs.

Most other backends have already converted, and this patch converts the
DRM-backend to the libweston-style init API.

The libweston-style init API includes a header for each backend (here
compositor-drm.h) defining the configuration interface. The compositor
(main.c) prepares a configuration struct to be passed through libweston
core to the backend during initialization.

A complication with the DRM-backend is that outputs can be hotplugged,
and their configuration needs to be fetched from the compositor
(main.c). For this, the config struct contains a callback member. The
output configuration API is subject to change later, this is just a
temporary API to get libweston forward.

As weston_compositor's user_data was not previously used for anything,
and the output configuration callback needs data, the user_data is set
to the 'config' pointer. This pointer is only used in
drm_configure_output() in main.c.

[Bryce: lots of stuff and rebasing]
Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Benoit Gschwind <gschwind@gnu-log.net>
[Pekka: write commit message]
[Pekka: squash in "drm: Don't hang onto the backend config object
post-backend_init" from Bryce Harrington]
[Pekka: drop the compositor.h hunk]
[Pekka: do not #include inside extern "C"]
[Pekka: remove incorrect comment about weston_drm_backend_config
ownership.]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>