lib/lp-media.c: Handle the destruction of individual media objects.

Currently, we do not support the destruction of individual media objects---only the whole scene may be destroyed.  One possible way to cope with this is via some _lp_media_make_orphan() call, which could stop the media object and set its parent scene to NULL.

This bug also affects the Lua bindings.  It prevents garbage collector from unref-ing the media object in __l_media_gc().
