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>