The root transform of each hierarchy can be extracted from the first
transform of the list in the hierarchy, so no information is lost. The
main reason for the change is I discovered (obvious in hindsight) that
deleting root transforms was O(n) due to keeping them in an array, thus
the use of a linked list (I don't expect a hierarchy to be in more than
one such list), and I didn't want the transforms to be in a linked list.
Since transforms now know the scene to which they belong, and they know
when they are root and when not, getting the transform code to manage
the scene roots is the best way to keep the list of root transforms
consistent.
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).
