The backend headers are renamed from compositor-foo.h to backend-foo.h to
better describe their purpose. These headers are public libweston API for each
specific backend.
The headers will also be used like
#include <libweston/backend-drm.h>
instead of
#include <compositor-drm.h>
to give them a more explicit namespace.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>