Giulio Camuffo authored Oct 11, 2014 and Pekka Paalanen committed Jul 09, 2015

This is a preliminary change for libweston, with no functional modifications.
Separate the backends and the core weston_compositor struct, by creating
the weston_compositor in the main(), and having the various backends extend
the weston_backend struct, an instance of which is returned by the backend
entry point.
This enable us to logically separate the compositor core from the backend,
allowing the core to be extended without messing with the backends.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>