Rename public backend headers

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>

remoting/remoting-plugin.c

@@ -44,7 +44,7 @@
 #include <gst/video/gstvideometa.h>
 
 #include "remoting-plugin.h"
-#include "compositor-drm.h"
+#include <libweston/backend-drm.h>
 #include "shared/helpers.h"
 #include "shared/timespec-util.h"