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/renderer-gl/gl-renderer.h

@@ -28,6 +28,7 @@
 #include <stdint.h>
 
 #include <libweston/libweston.h>
+#include "backend.h"
 #include "libweston-internal.h"
 
 #ifdef ENABLE_EGL