meson.build, src: update for weston 12

meson.build, src: update for weston 12

Update dependencies for libweston-12 in meson.build, as well as
adjust sources due to libweston-desktop/libweston-desktop.h
moving to libweston/desktop.h

This adds support libweston 12 which includes the following changes:

- use MODULEDIR, for both weston and libweston when loading
- use backend, renderer when starting up and pass those up

Bug-AGL: SPEC-4578

Signed-off-by: Denys Dmytriyenko <denys@konsulko.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I269e877ee3ae8cf8f1447bda05e11422244a416e