Every wp_object has a nullable reference to the shadow structure
for a file descriptor. As the existence of wayland protocol
objects is not guaranteed in the face of compositor/application
misbehavior, when e.g. a wl_buffer is created from a wl_shm_pool,
the wl_buffer registers a reference to the file descriptor that
the wl_shm_pool had. Methods that imply a file descriptor change
(wl_keyboard.keymap, wl_surface.commit) mark the object's referenced
file descriptor as dirty, so that changes can be transferred.

This commit does not yet implement any deletion logic.