libweston: Introduce backend.h

Introduce a new private header file that only internal backends are
allowed to use. Starts by migrating functions that operate on the
'struct weston_head'.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

libweston/backend-drm/drm-internal.h

@@ -55,6 +55,7 @@
 #include <libweston/weston-debug.h>
 #include "shared/helpers.h"
 #include "libinput-seat.h"
+#include "backend.h"
 #include "libweston-internal.h"
 
 #ifndef DRM_CLIENT_CAP_ASPECT_RATIO