compositor-rdp: refactor configuration API

Implement a "well" defined API to configure the rdp backend.
Following according to discution about libweston API.

Signed-off-by: Benoit Gschwind <gschwind@gnu-log.net>
Reviewed-by: David FORT <rdp.effort@gmail.com>
[Pekka: added missing headers to Makefile.am]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
4 files changed