This node is a bit special in that it uses two internal sockets
for a hack for Eevee; see rBffd5e1e6acd296a187e7af016f9d7f8a9f209f87
As a result, the `SOCK_UNAVAIL` flag is exposed to socket builder API.
Reviewed By: JacquesLucke, fclem
Differential Revision: https://developer.blender.org/D13496