compositor-wayland: split backend_init
compositor-wayland: split backend_init

Extract configuration parsing from init_backend function. The new
init_backend start by generating the configuration structure from
configuration file with separated function and then use this structure
to load the backend.

Signed-off-by: Benoit Gschwind <6d0fcc95e39ab8ccb88bafebb2075e9d5c92b713@gnu-log.net>
[Pekka: rebased]
Signed-off-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

sir-murray/weston,Gnurou/weston,giucam/weston,Gnurou/weston,krezovic/weston,sir-murray/weston,giucam/weston,krezovic/weston