libweston: Introduce libweston-internal.h

Introduce a new private header file that only internal parts of the
library are allowed to use and shouldn't be exposed in the public header
of libweston.

Start by adding by adding functions that operate on the 'weston_buffer*'.

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 "libweston-internal.h"
 
 #ifndef DRM_CLIENT_CAP_ASPECT_RATIO
 #define DRM_CLIENT_CAP_ASPECT_RATIO	4