It copies an entire hierarchy (minus actual entities, but I'm as yet
unsure how to proceed with them), even across scenes as the source scene
is irrelevant and the destination scene is used for creating the new
transforms.
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.
