Support using the `Vertex` trait when variable names are not known

[Rua]

#2334 ran into a problem where a shader has been optimised, and all its variable names stripped out. Vertex and co currently rely on having the names available, and will panic if they're not present. It would be useful if Vertex could specify not only names, but also location IDs, so that it could be used even when the shader doesn't have any names in it.