This actually has at least two benefits: the transform id is managed by
the scene and thus does not need separate management by the Ruamoko
wrapper functions, and better memory handling of the transform objects.
Another benefit that isn't realized yet is that this is a step towards
breaking the renderers free of quake and quakeworld: although the
clients don't actually use the scene yet, it will be a good place to
store the rendering information (functions to run, etc).
Holding onto the pointer is not a good idea, and it is read-only as
direct manipulation of the world matrix is not supported. However, this
is useful for passing the matrix to the GPU.
